Mmm, that's a good point seeing as most Mac-obsessed anti-M$ people can only seem to bring up flaws in Windows that were a problem in Windows 95.
OS X runs fine on G5s and all of the Intel-powered systems that I've used it on. Every G4 and under that I've seen run it, however, is slow as dirt. My processor is a pretty stripped down model; it's basically the Celeron-equivalent for the year that I bought the system (January 2002) with less cache, but it's still better than any G3, and even early G4, on the market.