I go 7

Due to compatibility with some softwares we use in inhouse sales agency, I passed just now to new Microsoft OS, 7. First impressions are: more speed in start and shut down system, absolutely better usability for Taskar (now beautiful and a really useful tool).
What's important is that is not just a style refresh but you can really feel the difference with Vista and you'll not regret to abandon XP. It's really a new experience even if not so much to force you to restart your mind to adapt to new procedures like if you go Mac.
I like 7!