i used XP til about a year ago, vista ultimate since then. 7 is essentially a service pack for vista, re-branded and overpriced. if you already have vista then don't bother with 7 (unless you can get it at a discount). if you have XP it gets a bit more complicated...
is XP faster on slow hardware? yes.
BUT... vista/7 is better on newer hardware. i recently bought a netbook that came with the crippled 7 starter edition (so castrated that it won't even let you change the desktop background), so i installed XP on it instead. after 1 year of using vista on my desktop it felt like stepping into a time machine.
vista is smoother somehow, more responsive if you have the RAM/CPU power, and looks a lot more modern.
then there are games, more and more of them support DX10 graphics, and DX10 only runs on vista/7.
so if you're a gamer and have a good machine you should consider the upgrade. get a friend's external hard drive to copy "files" (that means porn collection, right?) that you wanna keep and take the plunge. you won't regret it.
if, on the other hand, you have an older machine (and less than 2GB RAM), and your primary use is surfing the web/watching videos then save yourself the money and stay with XP.