I have been using XP for about 2 months now. It is by far THE most stable, and well put together OS I've ever used. If you're using WIN 2000 I wouldn't run right out and grab it, but if you are on 98 I certainly would. It really works well.
Advice. Stay away from "upgrading" your current OS... load it from scratch for optimum stability and a reduction in headaches as you sort out driver issues. Upgrading an installed OS has always been... well, it kinda works..lol My last bit of advice, make sure you have enough RAM. I would say 256 at least. (ram's super cheap right now btw)