WARNING: Linux talk ahead.
What with Celestia moving to a qt4-based approach, I decided to check out the new version of KDE. Some history here: my first Linux experience was with KDE on OpenSUSE. Eventually I found that KDE was not to my taste and moved to GNOME, then to Ubuntu when I got sick and tired of various stability problems in OpenSUSE (I believe the OpenSUSE version I was using is notorious for this kind of thing, however the very slow package manager was the final straw). On the other hand, while using KDE as my desktop didn't work for me, I am all for using KDE applications: apart from using the KDE interface for Celestia (the GNOME one not being so developed), I also use the Amarok music player as my default.
While Ubuntu "Hardy Heron" does not include the latest KDE4 in its default repositories, it is possible to add a repository which provides KDE4.1.2, so I used that. I must say, despite my long-standing aversion to KDE, KDE4 seems to be pretty good - it seems to be more sensibly-organised and much easier to get it into a state where it fits with the way I want my desktop to work, and the default look is much improved. I found a few annoying graphical glitches (these seem to particularly plague NVIDIA graphics cards), but these mostly improved after downloading the latest NVIDIA drivers.
The main problem I found with KDE4 (and qt4) is the font rendering: I prefer to use subpixel antialiasing with slight hinting, however in qt4, turning on subpixel antialiasing means full hinting is switched on regardless of the setting in the configuration box. I find that full hinting looks very ugly, so the best approximation to something usable I can get from qt4 is to use grayscale antialiasing, which doesn't look as good. Apparently this issue will be fixed in qt4.5.
I also tried out XFCE. This environment turned out to be a bit tricky to configure: you can't drag and drop program launchers onto the panels from the menu, which made setting up my big block of application launchers fairly tedious. However after doing a bit of work, I managed to get it to look very similar to my GNOME desktop. The advantage of XFCE is that things run noticeably faster than they do under the GNOME desktop: usually under GNOME and KDE there's a slight flicker as menus come into view as the drawing process is slightly too slow, despite me having a not particularly old machine. The Thunar file manager is also pretty nifty. This does not appear to occur under XFCE. Even the KDE programs appear to run faster. Since I am not one to go for fancy desktop effects — one of the first things I do to a desktop is to switch these kind of things off — XFCE makes a perfectly good substitute for GNOME.
In fact, I like XFCE so much that it has replaced GNOME as my desktop environment of choice. I still use GNOME programs for certain tasks: in particular, the default XFCE text editor Mousepad is too simplistic for my needs (for a start, there's no syntax highlighting), and the image viewer Ristretto doesn't support seem to support left/right arrow keys to navigate between images, so I'm using the GNOME programs gedit and Eye of GNOME instead. However when the next version of Ubuntu is released, I'm going to be downloading Xubuntu instead.
So, all in all, despite it not being one of the big two desktop environments, it is well worth giving XFCE a shot, particularly if you prefer your desktop environment to be functional and not get in the way. You might find you like it.