I am quite impressed with win7, i put it in my laptop and it was like an instant upgrade. Games that i used to play that struggled to get 20fps now easily get 30 fps with some spikes that go much higher. The OS seems to respond faster, though sometimes it can take a while when programs are starting up (this was also an issue with vista) the wireless setup is nicer than vista. (though I wish it was as easy to navigate as xp, but it's performance is overall better.) I haven't tried XP mode yet even though I have Win7 pro and my laptop supports it, havent really had a need to all my programs seem to work just fine under win7. the only issued i have is some of the drivers are unstable (probably because they are still in beta), but over all it works great.
Oh and one last thing the win7 task bar takes some getting used to it isn't as fast to navigate as older taskbar however it does have some advantages, such as it doesn't get as cluttered as older task bard and is a little more configurable.
So far this is the first OS Microsoft has come out with in a while that is worth buying especially when it cost me only $30 thanks to being a university student.
My free upgrade came a while ago. It's an awesome OS main one i use now.