XP still gets the job done, I have it on my older laptop and it did everything I needed it to.
My main system is still rocking Vista, I installed it the week Vista was released and am still running the same install (it even survived when my motherboard died and I replaced with a different brand). My XP installs never lasted this long, they usually slowed down and I had to reinstall eventually, my Vista install has been speedy the whole time (although after this many years I've developed a couple software issues that I am too lazy to fix).
My new system (laptop) is running Windows 7 x64 (as well as the majority of our BCCHardware test rigs). Everything is speedy and works great, no complaints as of yet.