I think it's generally accepted that Win7 is a superior product over Vista, but hotly debated over whether Vista is a superior product over XP.
A few years ago, my big draw to upgrading to Vista was DirectX10, better x64 exposure (I ran XP Pro x64, and... bleh,) and the massive improvements to memory management.
Some will say "oooh, Vista uses a ton of memory even when it's idling," but the important thing to understand is that's what an OS should be doing, so jobs are rapidly cleared out of CPU time. Ideally, an OS should always be using memory.
99% of the flak pointed in Vista's direction is poor driver support at launch time. Hardware vendors have caught up over the years in driver support, and that has basically become a non-issue. The rest of the issues are just garden-variety user complaints, which you'll run into with any Windows operating system. "Boo, Windows updates broke my Vista box." Yeah, welcome to Microsoft land. If you haven't seen a Windows operating system killed by vendor updates, you haven't been using computers long enough.
Finally, any comparison between Vista and ME is stupid. When you're running ME, you're living a life where you reboot your PC with no real confidence that you'll ever see your desktop again. Even seasoned IT guys - who can rebuild their OS from the recovery console - are afraid to run ME. There is no comparison.
Anyways.
Win7 is basically everything I like about Vista and more, and the performance on equivalent hardware is better, supposedly. (I haven't tried this firsthand and have only used 7 for building VDI at work.) If you can afford it, buy it. If not, no big deal. Vista is pretty swell too.