I don't know if anyone else uses them, or if most people stick with what WinXP provides (or maybe don't even run XP) but we've just reinstalled XP on my g/f's computer and were looking for some suitable new style, and boy is it hard! Most of them are either Mac clones (and probably clones of Mac clones!) until they're coming out of your ears, or they're such terrible colour combinations that it'd burn the image into your eyes or give you the worst migraine ever (see here for a film-inspired sample and here, here and here for ones where the colours are just awful for normal use)
I'm using a theme called FatalIE, which is nice and clean and simple, and without the garish colours or FPI (Fisher Price Interface!) bits of default XP skins.
Is anyone else using anything? or does anyone want to know how to do it?