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.