(short: my life with kde4 and after office politics holocaust)
A few tricks of fate have conspired to place a large office, two fairly powerful (a dual P4 and a dual Xeon, so both i386 and x64) machines, and four flat panels (two per each; castoff from other people upgrading) in my hands. I've got a desktop of something like 7000x1100 resolution. It's running Kubuntu Hardy, with the KDE4 goodness attached to it. Never really guessed I'd wind up with such a nice environment. I'm generally the park-it-in-the-datacenter type, and I'm happy to poke away at my laptop rather than bask in wall-to-wall displays (Steve, if you read this, they asked that I move in, it wasn't something I asked for...). But, with the machines and the displays, I can get a whole lot done and monitor a lot of different systems at the same time. Stuff I don't trust to the GUI I can just run in screen.
As for the GUI, there are a few tiny bits and pieces that are broken. Scrolling seems to have its fair share of issues, and it's exploded once in the last two weeks of usage. I also had an issue for a little while when KDE3 parts were running in the KDE4 environment (I added KDE4 after a vanilla Kubuntu Hardy install — to both machines). But in general, it's stable enough for normal usage, and because there's "really" Unix under there, all the stuff I usually ask MacOS to do (and am pleasantly surprised when it does...), it does without complaining, and even generally the way I expect it to.
I said previously that it was close to as pretty as MacOS. I think that's misleading. It's got that very "robust" look that Unix window managers have always had (going way back to IRIX and the early CDE displays and forward to black/open/fluxbox), only there's a fairly nice shine on it. It's not prettier, but in its own way, it's not less appealing, either. It's just different. And, frankly, if this where they are now, Apple should keep its eyes on these folks, because the next few bumps are going to be really nice.
Ready for the desktop? No. I spend a fair amount of time in #kubuntu answering questions, and while I mostly remember asking these questions myself ten years ago, the fact that people are still asking the same questions, today, is an indicator that where it counts — the stuff you don't see — is still a large pain in the ass. Stuff like sound configuration, the fact that your window manager is actually composed of lots of little packages, that it's not really clear (to a newbie) what the difference between "Xorg" and "startx" are, or even when they're in a terminal window in X or whether they're attached to the (k)onsole. Will it ever be ready for that crowd? I'm not sure it will be. It's my hope that there are enough people out there like myself, who drive a need for a Linux (well, a Unix anyways, I'm sorta ambivalent about Linux as a whole, but I gots my preferences) on the desktop, and that there are people out there developing this resource. I'm not sure what's in it for them, other than they like writing software that people use (that's one of the most fulfilling things I've ever done for a living; however, generally my end of the bargain is backend and infrastructure rather than interfaces — other than APIs — and user-facing stuff).
As usual, patches welcome.