I've found Windows 7 to be a very welcome upgrade from Vista, although I had very little problem with Vista apart from the amount of resources it used up, particularly battery life on my laptop. I stuck with it, and loved it.
I got Windows 7, along with my sister and we both love it. We both think the new task bar is a really good idea, and integrates really nice and smoothly with Windows and all the programs.
My sister has never used Vista, so she made the jump from XP, and she said it feels a lot quicker, and it looks fantastic.
I've had Vista on my laptop and did a clean installation - boy am I impressed! Boot times are 100% faster, and the amount of resources it uses is back down to an "acceptable level". It's given me an extra hours battery life over Vista, so I get nearly 5 hours (wireless off), 4 hours (wireless on) respectively - which is a fantastic improvement.
I cannot see myself going back to Vista, although I did like it - just a little to heavy for my liking, but still a fantastic operating system. Windows 7 is amazing, so clean, polished and it actually feels finished - not like a DIY job - always a few bits left over at the end.
My girlfriend said she hated Windows 7. I said spend some time using it, low-and-behold, an hour later she says she actually quite likes it - so that's another copy that needs ordering.
I've not had any experience with upgrading an install, I never recommend it to anybody, but clean installs work perfectly - nearly all my hardware was recognised straight up, with the exception of a few older pieces of hardware - an amazing improvement!