I'll give you some first hand experience from the release candidate. I do have very limited experience with Vista though.
I like it. It's smooth, out of the box it's neat and clean without a ton of stuff cluttering up the desktop. I had some trouble with some drivers on my pretty old PC, but that's not MS's fault.
Getting into some technical aspects here. I really do think that everything that makes 7 different from Vista could have been released in a Vista service pack. I know there will be a few people in this thread that will say they'll wait for 7 SP1 before they get it. Don't think of 7 like a fancy schmancy new unstable, untested product from MS, but Vista SP2 with a different user interface. I think the whole reason MS decided to just release all this as a new operating system is because they want to get away from Vista as soon as possible.