I've been a Vista user for the past year and I've hated every day of it. It sucked up a full gig of RAM, had some pointless and downright baffling features, required constant upgrading to keep it from dying on a daily basis and turned the highest spec PC into a lumbering dog. Thank Gates, then, for 7!
Well, sort of. It does have its niggles: awful sound effects; doesn't support Outlook; peripheral drivers (and iTunes, bizarrely) need reinstalling. But these are minor. Easily dealt with and by no means deal breakers.
What might be a deal breaker, however, is the fact that, aside from its fancy new taskbar (which is fantastic once you get used to it), 7 is little more than XP with some graphical tweeks. Almost everything that differentiated Vista (and made it slow and pathetic) has been removed, but seemingly nothing has been put in its place.
Again, this isn't necessarily a bad thing, but it begs the question; if you're running XP (or have the ability to 'downgrade' from Vista), is it really worth forking out the cash for a new taskbar? I'd say not.
On the other hand, if, as I was, you're stuck with Vista, do not hesitate to upgrade to 7. It's simpler, smoother, faster and better in every way.