I was a beta tester for Windows 7 (in fact, all the way back to Whistler; I missed out on Chicago) and this is an honest review of what I think about Vista, Windows 7, and XP, with 7 being the primary focus. This review assumes you are familiar with XP.
Windows 7 is a major upgrade from Windows XP, if this is the route you are taking, you missed out on a lot of new features brought about by Vista, and you might be a little overwhelmed at first at everything that has changed in 7. However, if you were a Vista user, then the only things to really take note of, from a users perspective, is that the Start bar has received a major reworking. Getting familiar with the new start bar is pretty easy and will come naturally after a few days for most people.
Under the hood, as they say, is what most people are interested in, because "Vista was so full of bugs, etc." However, I've used Vista extensively with little to no trouble. Vista's main faults were driver signing enforcement (users who were early adopters, found that they didn't have good enough hardware and drivers to match), and a new strange user interface. Networking also went from simple in XP, to quite a pain in the rump if you didn't have a solid understanding of networking. This upset quite a few people, because XP worked just fine, and Vista ran really slow on their computer and wanted to eat up all their ram, resources, etc, and they couldn't set their network settings as easily.
Enter Windows 7. It's Vista, with all the patches, and instead of just assuming everyone has a super beast computer, it adapts resource usage based on your hardware. So install it on a Super Computer, it will actually utilize the resources available (unlike Windows XP, but just like Vista). Likewise, place Win7 on a netbook, and it will become very conservative with resource usage (like Windows XP, NOT like Vista).
Other than that, Win7 is very much like Vista
Link to original Review
Green Complex IT Guides