I decided to make the jump to lightspeed...er... linux this year fully after hedging around it for a long while. I've used windows and mac both since ... well forever. heh. I burned probably a dozen different linux distros and booted live cds, and I've probably installed 4 or 5 onto the actual drive to continue playing with it. I really wanted to like KDE... I kept hearing how nice it was, how the eyecandy was great, how the apps were great etc. etc. etc. In screen shots and reading it sounds great... in practice, I had the opposite impression of KDE 4. I found that the eyecandy was lacking integration (not talking compiz here... that was harder for Me to get running than on gnome which is what I'm using right now with Ubuntu 8.10... plenty eyecandy there, but that's nothing to do with the actual distro since it's 3rd party apps).
There are some nice features that I liked.. the integrated widget layer akin to konfabulator (coming from using OS X as a primary OS, you get used to having widgets for everything). I also like a lot of the K-apps... they did a nice job of integration on their basic apps... once you get past the learning curve... this isn't windows or mac, so it takes a little bit of learning... which is fine... that's why I switched anyway heh. I gotta say... having the "desktop" be a window is rediculous though. The concept of a desktop is that it's well... the desktop... it's not part of the file structure in it's interativity. Having it function as a widget folder was just plain stupid IMHO... we ditched that functionality in windows 3.1 if anyone remembers... and good riddence.
The famous "configurability"... honestly... I had a hell of a time making the couple of simple tweaks that I wanted to make. Things like I wanted to take that stupid widget swirl in the upper right and move or hide it... never could do that. Moving the main panel to different locations on the screen proved more difficult than it should have been. These were all things that gnome, os x, and windows all let me do pretty quickly within their own methods (gnome is particularly easy with it's panels).
Little things like having the bouncing icon on the mouse to tell you what is loading are cute and I can see where they are nice... but I eventually got sick of it and wanted to turn it off... couldn't find that either.
A complaint I've had about all the linux distros I've used to far is that I would like a centralized hardware device manager tool. KDE is no different about this... couldn't find anything. I don't know if that's the DE or just linux though.
Over all I've installed it a couple times and run it off the livecd a few times... I really want to like it because I agree that it's "forward thinking" and could be great... but in practice it just feels... broken. I'm all for innovation, but I think sometimes in linux development they think "well... we want to not be mac or windows... so we're going to do it different"... which is nice... but sometimes the conventions that are in mac and windows are there because they work. It's a fine line. Linux does give us a lot of choices of how to tweak things (once you figure them out)... certainly more than any other *modern* OS seems to.
Diehard hacker that I am though, I'm planning on reinstalling KUbuntu on some unpartitioned space I have left to play with it some more.
$0.02