Used them all at one time or another, and I have a couple unpopular opinions. The first is that (contrary to popular belief), XP is not God's Gift to Desktop Operating systems. XP at it's release was much worse than Vista at it's release, and that's a fact. Less driver support, more bugs, and issues with drivers (*still) can bring the entire thing down. The advantage XP had at it's release which Vista did not is that the OS's XP replaced were even worse - with the notable exception of Win NT. So (we) dealt with it since it was still an improvement in the (desktop) environment it was intended.
The second unpopular opinion is that if you can't get Vista working properly, then you need to turn whatever passes for your Technical Credentials in at the door on your way out. It's not hard. "Different"? Yes. But not difficult.
With that out of the way:
I've said it before: If you are looking for "XP 2010", then you are going to be deeply disappointed. Win 7 is not XP. Never has been, nor will it be.
As it stands now - I am using Windows 7 RC (build 7100 - not the leaked stuff), which I installed via an in-place upgrade of an image of my Vista 64 drive. It's been reliable and fast since install.
Likes:
I like the Action Center - It could be better, but it makes it easier to figure out what's going on with your system easier than in any previous version of Windows.
Superfetch has been toned down to be more polite about how it manages and caches data. It still does does it's thing, but isn't nearly as aggressive about it. Preferring instead to work in the background at a lower priority. The result is that the desktop becomes responsive much quicker on start up than it does under Vista. This is an easily noticable difference.
7 is better about how it handles multiple program windows - In earlier versions Windows keeps a copy of texture data for each window. Where possible, 7 uses a single version instead. So resource usage in situations where you have a lot of windows open is improved.
UAC is configurable, and the default is more polite to begin with.
Search - I've said it about Vista, and it holds true in 7 - I like the search. Indexed Results come up as fast as you can type the name. No More Navigating Menus and Sub Menus!
There's also a nice, and easy to use color calibration utility, better SSD support, and it Defrags itself.
I have a slightly more mixed impression of the new Taskbar - I don't like the defaults (icon only in a square box) all that much. Easy to fix using the context menu (right click). And if you're a Quick Launch junkie (like me), the defaults get really crowded, really fast. On The Other Hand: If you have a widescreen monitor - Try setting the Taskbar to the left or right side, open it up a little, and it quickly becomes superior to the older versions. Arrange your commonly used programs how you like, you'll have readable text instead of an icon, and the same program stays in the same place - always. So with a little practice, muscle memory becomes the rule when you want something. Nice...