I actually do on my workstation - starting from KDE 3.10. In times of godawful 4.0-4.1 I considered changing DE, but starting with 4.2 it was somewhat usable, so I finally upgraded to 4.3, and starting from 4.5 I rarely encounter any problems. Of course, many of KDE components are disabled on my system, and DE is customized to my needs pretty hard (plus I am one lazy guy, and much of my work tools are scripts of some sort, so I don't use many of KDE's native programs). But once in a while I like to rearrange my desktop - it doesn't affect my workflow, but brings some sense of refreshment. So, long story short, it's usable and fast enough for my needs - when I tried to work in Gnome (version 2 at the time) and XFCE I was somewhat disappointed by lack of customization in them.
Still, I like that diversity - some of my friends use Gnome or Unity and are pretty happy with them, some are Mac users, some are still using Windows 7 (I use it at home for gaming). I don't personally know anyone who would like Windows 8, but I don't mind this system too. Anyone can find something to their tastes, and I can only approve of that.