KDE's current version is outstanding. We could spend all year talking about history, but KDE4.8 works pretty well, and frankly is a great option over Unity, Gnome and some of the lightweight desktops if you value functionality over light weight. If you like lightweight, don't go with a desktop, go with a window manager.
Incidentally, you can tell someone who hasn't really used KDE by comments like "it lacks refinement" or "it isn't pretty" or "kde4 is slow". There's really not much I can say for people who say "I don't want to customize my desktop" as the default isn't bad and KDE's biggest feature is it's customization capability.
That said there are two components that need to be better explained and left to the user to decide if they want them: symantic desktop and Akonadi. Symantec desktop (nepomuk) is basically text search engine and tagging toolkit that lets you rate, comment on and tag files. The search engine works now, but for people with networked home directories, it is not the right answer. Akonadi is the backend for the personal information manager applications. If you are not going to use Kontact (the KDE outlook clone), Akonadi probably doesn't need to run. If you are using Kontact, Akonadi offloads sending/receiving so the front end applications can be light and fast.
I'm a python developer most of the time these days I use emacs, Wing, iPython, yEd (for charts and process diagrams) and do some documentation and proposals in LibreOffice. There are a few that have been part of KDE for a long time that make it especially nice:
* opens a terminal in many apps. Handy.
* KIO - allows you to open files pretty much anywhere without the need to mount drive. You get very used to being able to open and save files on all kinds of remote systems and services from the highly functional file save/open dialog.
* Dekstops and workspaces - multiple desktops and multiple dashboards. Most are an away.
* Plasma Desktop - You can pretty much customize it however you like. Want a start menu and panel ? OK. Want a mac like menubar? OK (xbar) MacOS like dock? OK. Mac style dashboard? Got it. Windows style widget bar, ok, you can do that. Want a quicksilver like launcher? (that's been there for almost a decade). Want files on your desktop? OK. Want remote files on your desktop? OK. Don't like the look? Change it.
* Konsole, the KDE terminal app just works. And has a ton of features with an easy to detach tabbed GUI and some pretty nice automation features.
* If you did a file manager shootout, it would probably finish Dolphin, Konqueror, Finder, MS Explorer, Kommander and everything else. KDE's file managers give you a lot of flexibility and outstanding integration with tools. Dolphin is designed for ease of use, Konqueror is an MS Explorer style kitchen sink and Kommander is a Norton Commander style app. All leverage KIO to be able to browse remote systems as if they are local and launch background tasks to move files around.
* Amarok - Music player. Very well done. Probably the best one out there short of iTunes...
* Kmail - A very well done feature rich mail client.
Is KDE perfect? No. KDE went through its rearchitecting four years ago, and has emerged to be very, very good.