Well, so far, I like Windows Vista. It has some nice, new features, and nothing really horrible. Boot up and shutdown seems to be much faster - but maybe that's because I have a new, faster computer. I really like Office 2007 - but it also runs well on Windows XP.
I don't think I'd buy an operating system upgrade, but it did help me decide it's time to buy a new computer - a year later than I had planned to buy one.

-Joe-