I was a long time KDE user from 1.X through 3.X, and waited until about 4.1 to try the 4.X series. It was a disaster, with huge stability problems with dual monitors, an unpolished look and feel, and general slowness (despite turning off animations, desktop effects, etc.). GNOME2 + compiz by comparison was polished, reasonably customizable (something I never liked about GNOME was that in many versions things were hard to tweak), and fast! My Intel chipset doesn't have good 2D performance; ironically, 3D acceleration of window drawing via compiz made things like desktop switching much faster (and more like my slightly older Intel chipset which always did fine with 2D...).
Fast forward to a few weeks ago, and my choice was the Unity interface in Ubuntu or the GNOME3 shell in the new Fedora. I realize I could stick with GNOME classic, but deprecated platforms will only be around so long. I knew from descriptions and screenshots that Unity wasn't my kind of desktop environment, so I gave the new Fedora + GNOME3 shell a shot.
Yikes. It takes simple to an extreme - right clicking doesn't do much anywhere. The panel up top is small and there was no obvious way to add anything to it, so it was generally a waste of space. The activities button produced a huge, ugly menu with giant icons and tiny text. it has the feel of a tablet environment but would be a disaster to use on a tablet as is (with some UI elements being finger-sized, others being tiny).
xfce in principle is decent, except that the 2D rendering on my chipset is slow enough that it made it feel slower than GNOME2 + compiz. I suppose I could run compiz with xfce as well, but compiz produces a bug with R's data editor that I was interested in eliminating anyway.
We're up to KDE 4.6 though, so I figured it was worth another shot (having tried 4.1 and 4.2 briefly). Turns out that the distribution matters - I've had a few less issues with Kubuntu than Fedora Core 15. In FC15, some GNOME apps like gnome-terminal would have their windows shrink whenever I launched them, very odd. KDE as a whole finally feels polished, fast, and powerful. I would choose is over an updated 3.X series. Its handling of docking and undocking with multiple monitors is still more tempermental than GNOME2, but manageable. And kwin for compositing lets me avoid compiz and its R editing bug. Best of all, I get back some of the things I really like about KDE - the file picker with kio slaves being the biggest among them.
The bottom line is that Unity and the GNOME3 shell are a huge step back for users who like a more traditional desktop with lots of customizability. xfce and KDE can step into those roles, and I think we'll see renewed interest in them.