Well, XP really shouldn't be compared to NT, but to ME and Windows 98. From each of those, it was a HUGE upgrade.
I've not played with Windows 7 much yet, but I've played with Vista on my main workstation at work. Vista isn't bad, and when used appropriately, it isn't unstable. But it is EXTREMELY cumbersome. And there are a few places where Microsoft tripped over themselves in releasing inferior code to that of XP, and a couple of cases where they didn't have a Vista solution while an XP solution existed.
My most major objections to Vista are: 1) change in the way reverse DNS zones are broken down (VISTA PLAIN SUCKS AT THIS), 2) unavailabily of certain drivers (most notably, my Kensington Expert Mouse Trackball), 3) delayed release of Exchange tools, 4) lack of support of older DFS tools, WINS tools, etc., 5) difficulty and more levels involved in getting to things such as my network card settings, and 6) inability to run WordPerfect 2000. (Okay, that last is being unreasonably picky, but WordPerfect is still a far superior product to Microsoft Word, and as long as I can run it before being forced to upgrade, I'm going to.)
Stability has not been a problem for me, but if I had it to do over, I would NOT have upgraded - just to keep my old DNS tools. As it is, I run them in an XP virtual machine running in VMware Workstation on my Vista box.
I'll be glad to convert to Windows 7 - just as soon as I have the time required to do the build-up.