Personally, having tested them both side by side, I must admit I'm underwhelmed by Win7 compared with Vista SP2 - reminds me a lot of Xp vs. 2000.
Now don't get me wrong: Vista RTM was pure horse dung (the installer alone is horrendous, in its "You want to delete partition 2? Let's trash partition 5 instead!" way).
It's just that SP1 fixed many problems, and SP2 fixed even more - with all fixes and updates applied (the platform update and recent GPU drivers), Vista performs as smoothly as Win7, if not smoother. You can also revert it to XP-like themes and services running, which has a rather drastic effect on RAM use.
On the other hand, Win7 feels like 'Vista SP2 + lipstick' - it's not BAD, and I recommend getting it if you need a Windows version, but those of you who got Vista (I went Linux when they started writing 'XP or better' on software boxes), don't feel so shafted and keep your money.
The thing that makes me dislike Win7 is that, as said by several people in previous comments, advanced settings in Win7 are even better hidden than in Vista.