Been using Windows 7 for about, 4 months now. And I am very impressed about it.
I would say stick to XP, but if you already got Vista, go for Windows 7 if you want to use an Windows operative system. Windows 7 kicks windows Vista totally.
It is definetely more troublesome to downgrade from Vista to XP than Vista and upgrade to W7.
Been helping my friends also, who had issues with Vista, like games working bad, very slow, eating to much resources.
After using W7 Rev 7k all those problems got fixed, and now after getting rev 7.1k, it seem to be even better.
Also you don't have to think about your computer drivers at all, if you got vista, windows 7 will take and use for an example your graphic card driver, audio driver and so on from windows vista.
You can also let W7 examine your computer, and search for drivers updates which fits for your hardware.
I love the new taskbar system. Especially with shade grid function.
Windows 7 is still a bit unstable, especially unofficial rev like 7064, 7022. Im testing 7100 now, also rev 7000 which is the official beta is a bit unstable.
But the worst case that have happen to me, is that I get bluescreen while closing some firefox windows after searching youtube. Which happens very rarely.
Windows 7 pwns Vista. But my heart still belongs to tweaked XP.