My file server / web server / database server / second desktop has been FreeBSD for ages. It's currently running 6.1 (i386 - it's an old dual P4 xeon), with samba/NFS/PostgreSQL/apache+PHP and mod_python, with storage on a hardware mirror, and KDE 4 for desktop use (I wanted to test it, and then never got around to stop using it). A bit overloaded, but it works oddly well.
The newest desktop PC runs 8-CURRENT. I'm writing a thesis on it at the moment, so text editor + LaTeX and a web browser. KDE 4 - the exposé clone in kwin is really quite handy, and the nvidia drivers are good enough to keep it enabled.
It also sees some gaming - World of Warcraft runs fine in Wine. Sure, it's markedly faster on Vista, but it's still perfectly playable. A bit of DLL copying got ventrilo working as well.
Getting WoW working was apparently enough to keep me from dualbooting, as I haven't used the Vista install for a few months now.
My laptop runs XP, though. It's got some programs I know don't work in Wine (e.g. Lightroom), limited disc space (so dualbooting is inconvenient), and XP is better at battery life, wiFi and suspend/resume on this hardware.
As for
why I run FreeBSD, well. I did the usual round of linux distros some years ago, but ended up with FreeBSD 4.7, and I've sort of stuck with it for my toy server since. Using it as a desktop started when I swapped my "server"-hardware for something fast enough and found it convenient to do things on the always-on machine instead of booting the other (windows-)PC, and then I started keeping a FreeBSD partition on the other one as well, to play with it on a reasonably current computer. It sort of escalated from there.
Oh, and I just remembered. My router runs m0n0wall, which is technically FreeBSD as well.