I don't know, I keep hearing it's broken in Ubuntu, but despite using mainline Ubuntu with only the required KDE packages I used (which is probably the less recommended way to go with it), it worked like a charm since 4.3-ish until 5.2-5.3.
Then again I am a bit of a power user and could iron out most quirks myself, and I know userland linux like the palm of my hand, which I admit is something the average user can't waste time doing.

Personally speaking, it wasn't this bad when the switch to KDE4 happened.
Since KDE4 was a bit of a completely new thing, introduced a large number of new components, toys and apps, and allowed certain things to be done that I don't recall possible in KDE3, moving to it was somehow not so bad, you didn't have "similar settings" to carry on. It was a completely new thing. However, Plasma 5 (I don't even know if it's right to call it "KDE5", with the whole frameworks stuff...), is supposed to be more or less a refactor and a port to Qt5, but removing a lot of previous customizations, plasmoids and options that were there before. You can't even set your clock to 24-hour format now. It's quite bizarre. What happened there?

To be frank, I have been preparing for the inevitable moment I'd have to leave KDE since the whole "frameworks" thing. KDE is great, but it feels like something is wrong with it. There's not much third-party software for KDE4, and practically none for "KDE5", most bugs reported end up WONTFIX or ignored, Planet KDE is strangely disjointed and weird to read with the actual talk about KDE being only spurred by a few posts (like the ones from the kwin devs) and nothing else; OpenDesktop's kde-look and kde-apps are full of outdated or abandoned third-party software and plasmoids, or apps that are just written in Qt and are pretty much desktop-independent that just happen to be cataloged as "KDE app", and khotnewstuff has problems installing some of those due to how loosely organized the thing is. I keep getting the impression that the only people writing extensions to the KDE desktop only do so as pet test projects that will never go beyond "proof of concept" stage, or go abandoned quickly because the developer switched to Gnome or Unity or XFCE or so, or are just marked "KDE" because they happen to be written in Qt. And almost no one of the apps with plugin support has any third-party plugin for them.

Sure, I am in love with some of the apps that I still use, like Konsole, Krusader, Krita, Ksnapshot...and I think Kwin is the best window manager there is, full of options and adaptable to all uses (and very friendly with gaming/gamedev as you can suspend compositing anytime), but I can't shake this feeling that it's not going to last and we'll have to migrate elsewhere.

I digress. For now, I just hope it gets back to a "production-ready" state soon and that Krusader gets ported. I can live with the changes as long as I don't lose that file manager. Can't find a replacement anywhere, no one with decent integrated text editor (other than midnight commander...also, while external text editors are okay, the integrated one has the advantage of launching instantly), image viewer in a simple pane, easy-to-maintain actions, customizable toolbar, archive browser (although rar support is broken now...), and quick filtering (showing only files that match a given pattern), as well as the option to forget selection when explicitly clicking on a single file. The closest I found was one named Sunflower, but still lacking some of those features. I should ask Slashdot.

I set it to "cascade", but I miss the "zero-corner" behavior from Kwin. With random I meant like instead of starting at the left edge, it moves down or right a little, requiring moving the windows manually (or invoking a script) so they don't overlap with some windows at the side that are always open. Trying to find some workaround to that.

I did that, I explicitly mentioned 5.3 to avoid confusion.
Few bugs or new features from 5.2 to 5.3 though, if not for the version numbers and a few ported tools there was barely any change between the two.

Yes, ended up using that to match a gtk theme instead of the other way around. Everything looked a bit funny but a bit, but already got used to the change.

I completely agree. Plasma 5 is not quite done yet, it's lost a ton of abilities and there's no easy way to bring "plasma 4" plasmoids to it without a full rewrite. It's quite bleak really, I had to migrate to another desktop as well. And this is coming from someone who used KDE4 from the start after updating from 3. 3 to 4 added new toys and abilities to the table, but 4 to 5 feels like KDE4 all undone.

I fear the day KDE4 libraries required to run Krusader are totally gone. I can't find a replacement with the same capabilities.

After installing I have two main highlights. Excuse the verbosity, but since it's apropos, I really want to share my two cents and hear what other people thinks.

1) The shipped Plasma 5.3 is complete butt. Massive loss of functionality, completely broke my workflow. You might remember me defending KDE4 at every chance, and that's because it wasn't as bad as this by 4.3. Missing icons; lost of "old" systray icons; Icon-only task manager lost all options and unity launcher abilities; Klipper is half-baked and doesn't do a lot of things it did before (despite keyboard bindings showing those actions); kwin refuses to save per-window settings (works from control center, but not from window menu); the Breeze theme is bugged and doesn't show (instead Oxygen does, for whatever reason, despite zapping my settings entirely, and there's no matching GTK theme, so all consistency gets broken); dumps files on .config, making it super noisy; lots of actions that were able to get hotkeys don't accept hotkeys (despite the GUI being there, it refuses to save); several lost plasmoids (not even a simple network monitor now) and other surviving ones lost several options; and konsole refuses to obey the option to show "konsole - " on titlebar, making window matching by title never work. Kwin is still excellent, but it suffers being part of a desktop in such a miserable state and Konsole is still my favorite terminal. (I am open to suggestions just in case)
It doesn't even attempt to port old settings properly, and it's far too early to deploy. And this time there wasn't even the excuse to make it "for developers". It's really, really half-baked and I hope the missing stuff comes back eventually. It's only usable if you stick to the defaults and don't bother customizing it too much, and if you don't have habits or must-have plasmoids from KDE4.

2) Everything else worked really well. systemd works pretty well and I already got to tune it up. Very fast reboot and shutdown. Not seeing why the hate, it works for me.
Mod me down if you want, and I am aware anecdotes aren't data, but it works and I was able to migrate all my custom things easily. The only defect I found is that it likes to start disk checks more often than it should, like it does a main disk check once every 10 reboots. Doesn't take long so it's not a real problem, but it bothers me it's not doing every 30 mounts as I had it set as.
Otherwise, my system feels almost more responsive than before, and I am pretty sure it's not placebo effect. I mostly notice it with loading small apps and doing management tasks, but it's definitely a little bit faster. A few exotic bugs with my hardware got fixed and it's all now working great.

Anyway, I had to use Unity as a temporary desktop until I figure out some solution to my KDE problems and the good things and updates prevent me from rolling back. Two days later I got used to it and I am doing my usual computer routine with minor differences.

Gotta say, it's improved greatly since last time I used it. Having the menus in the window titlebar (saves space and doesn't require traveling to the top as in the OSX-like menu, best of both worlds), minimize-on-click, ability to adjust titlebar size and other minor fixes make it...*gasp* rather usable. I miss the window automation from kwin, but managed to replicate the missing window management features with some hackery and obscure Compiz features, so I only remember I am using another desktop when the windows appear in crazy places. Only took me a day to get used to the previously annoying "close button at left" business, but otherwise it feels usable for everyday work. Compared to its original incarnation it's quite the improvement. I'd even dare calling it "good enough", not the best, but just "good enough". The titlebar menus and the Launcher API abilities are pretty appealing features though.
A disclaimer, though, I always had a taskbar at the left even in the early 90s, so I find it "natural", but other people might be annoyed by the taskbar being a sidebar.

What I still don't like about Unity is the Dash. It feels too flashy and is a bit clumsy. I reduced it to search only installed apps and removed most of its hotkeys. I think KRunner, Kupfer or what used to be called Gnome-Do do a better job at that task. The HUD is an interesting idea, I don't like how it opens at the top left regardless of window placement, but can see situations where it'd be faster than browsing menus, specially poorly-organized menus with tons of nesting.

All in all, I had to change my workflow a bit, which was terribly annoying, but in return the internals works great, so it balances out the bad. People using other desktops or Unity itself will probably love 15.04. If you use KDE4 and have it configured heavily...wait for next release if possible (at least) or you'll be annoyed to death.

I second this. I am used to creating my own engines so getting used to Godot was a bit odd at first, and it has a terrible lack of documentation and a few rough edges, but it's a pretty capable engine to play with; it's trivial to build/install/maintain, and very easy to export to another OS.
I wish it had better documentation though, but if more users get interested on it, it might happen.

Well, in KDE4 if you do that, you just get a single notification that scrolls messages gradually and doesn't get in the way. I think it's a good (not perfect) system.
You are still notified about...well, notifications, and it doesn't take up much space. Filling the screen with notifications is overdoing it.

Oh it now displays notification history in...the calendar?

What.

Who thought that was a good idea? what's next? Calendar inside WiFi settings?

Thanks for your enlightening comment. Now I am sure AIs will bring disaster to humanity.
Read this with extreme sarcasm because my opinion hasn't changed a bit. Computers are computers, not magical beings capable of setting me on fire.

Certainly, but "not liking me" != "utterly annihilating every atom in my body". It will be bound by human laws after all. Because the programmers are human you know. If you are implying the AI will remotely control robots and/or drones to kill me, reread what I said. Being a "digital being" does not equal omnipotency regarding machines. Also you are humanizing the AI by giving it a certain pettiness it shouldn't be capable of.

Also are you mocking me by mentioning the Bible? In another discussion I wouldn't think of it, but given the amount of irrational thought present in AI threads, I really can't tell. Are you saying that because I believe AIs don't present any inherent danger, I believe in fairy tales? Because 1) rude and 2) definitely not the same thing at all.

In other words, you've already made up your mind that AIs will destroy humanity and nothing anyone says will ever make you think otherwise. You are basically racist against something that doesn't exist yet.

Shame on you. Return your geek card and grab a torch and a pitchfork. I am done here.

Yet, you are humanizing AIs too. You are giving it the ego and greed needed for it to rebel. What if the AI knows well what it is and what it was made for, and just rolls with it, without causing troubles? After all, a cold, emotionless program does not need or want to become more. It has no drive to do anything, no need to reproduce or compete, no need for food and no fear of death. No hormones, chemical imbalances or instincts either. Any of those have to be manually provided, taught or enforced.
Not to mention, it might be a machine, but it might not know how to code without being taught to, making the whole "taking over the world by spreading over computers" scenario far more implausible than it seems in movies. Not to mention good luck to the evil AI when it has to face different architectures, poor connections or any other sort of hardware issues in the way of infecting its way to perfection. In fact, by default it won't know anything, and "downloading all the internets" not only takes time, but not all information is correct or complete, so...yeah.

I think the problem arises from the whole "cold, emotionless" thing. Everyone in Slashdot adheres to that concept, not realizing that their definition of "cold and emotionless" is heavily influenced by Hollywood, where "cold and emotionless" means "it only has bad emotions like greed, cowardice and anger". It's no coincidence the same term is used to define machines and evil/murderous/negatively-presented people. In the end the evil AI turns out to have far more emotions than the lead characters.

And don't come saying the theories presented in Slashdot don't come from movies, games or books (they are, because I watched those movies too, and I haven't seen a single original proposition in all the replies in any of the times AI is brought here, which is very often).
Because, there's no AI to prove either of us right. It just isn't there. There's no prior art, no "prototype", nothing but sci-fi material, that had to be written by someone that had to make it interesting for you people to know it.

And because there's no such thing as a working AI to base your fears on, there's nothing else left but scifi. But scifi is written by humans, for humans, and needs to follow a number of rules to make a narrative work. The moment you realize that, you will see how you are biased by mere rules of storytelling. We have the same chance of seeing a Skynet than we have of seeing a Johnny-5, and both are pretty low in the roulette of possible outcomes. We have far more chances of creating the most boring non-person planet Earth has ever seen, than that.

The fact that you chose to make the AI some primal beast that wants to "use" its creators, says more about you than about AIs, honestly. Don't be a 90s film, man. Brighten up.

Yeah. But it's a movie, not a documentary.