I've been using KDE in various flavors of Linux ever since I picked up a copy of Yellow Dog Linux 2.something on eBay for the princely sum of $4.00. That was back in '01 or '02, if I remember aright.
Digressions aside, I've been using KDE for at least 5 or 6 years, on, let's see: the Pismo, the beige G3, the B&W G3 and the G4 Dual-Proc, on the Gateway 400 MHz P-III, the Gateway 900 MHz P-III, the Shuttle 2.8 GHz P-IV and now on my Dell Core2Duo 2.0 GHz, as well as installing various forms of KDE-using Linux on several friends' computers and both of my step-daughters' laptops. Several years on several platforms: I'd say I've been using it. And I've liked it. When I found out that KDE4 was coming out, I was actually counting down the day. Screw Xmas, gimmee KDE4!
Well, now I've got KDE4 - downloaded & tried it on the evening of the 11th, when it came out for general use. My take on it? It's great, and it really sucks.
It's great: it looks fucking awesome! Finally, all the compiz/beryl stuff works, something I could never quite get myself geeked up enough to make work before. It's fast, and it's the basis of something that looks like it's going to be really good.
It really sucks: KDE4 is, actually, KDE4.0. And what do we all know about software releases? That's right, avoid the x.0 release like a skanky hooker with a bad case of Kryptonian Gonorrhea. When it's all grown up, it'll be good, but right now it's a puppy that isn't house-trained. Here's a short list of the things I've seen so far that I don't like:
1: the panel is fixed in size, location and (as far as I can tell) color. It has a limited repertoire of widget/gadgets/whatevers that can be added.
2: the taskbar only has one option/setting: show tool-tips. I pretty much have to be able to turn off "show windows from all desktops" or it's not usable.
3: desktops defaults to 4. I use 8, and I'm used to using the ctrl+<desktop #> to switch desktops. I actually thought they'd removed this capability, because I couldn't switch from d/t1 (online) to d/t8 (dev) unless I clicked on the desktop pager. Turns out that you can only use the keyboard to switch desktops for the first 4 desktops.
4: back to the panel: it disappears completely. Not just to me, either. Check out the Kubuntu and KDE forums and it's happened to a bunch of people. The only cure it to delete ~.kde4, or, possibly, some file or combination of files inside that folder. Some people claim that all you have to do is delete the plasma-related files & you're good to go, but I had to delete the entire thing to get the panel back.
5: it's unstable. This is unforgivable on a weekly basis, much less 5 times in less than two hours. The only way to get out of freeze is to hard-shutdown by pressing and holding the power button.
Bottom line: I like it, a lot, but not nearly enough to use it yet. I keep checking for and installing updates (and there have been quite a few), but until it hits, probably, the magic x.1 roadmark, it's not usable.