I really, really like Windows XP. I have no use for Windows 2000 anymore. (There, I said it.)
I especially like the anti-aliasing of all on-screen fonts (dubbed “ClearType”). In this way, it reminds me of Mac OS X, but much faster (compared to 10.0 — I have heard great things about OS X 10.1).
Despite benchmarks and my stepbrother’s opinion, I also have not noticed that Windows XP is any more of a resource pig than Windows 2000 Professional; programs seem to run with at least as much responsiveness as before.