Well, I guess I'll be the contrarian here, but I really
like Vista so far!
I'm an experienced Windows user who has recently "embraced" OS X--I refuse to say I'm a "switcher" because I happily run both platforms, and both have their strengths and weaknesses. I've loaded Vista Ultimate on my new 24" Alu 2.8 GHz C2E using Boot Camp, and it runs fine. I don't find it to be particularly slow, though obviously XP is faster.
Most of the moaning you hear about Vista is the same as you heard about XP when it first appeared--"bloated" code (compared to Win2K or Win98), lack of drivers, compatibility problems, etc. These issues are real indeed, but to be expected whenever a major OS overhaul comes along. Vista will mature over the next few years, and these problems will become a distant memory. Of course, XP will be around for a long time, too. But, as time goes on, the compatibility balance will slowly tilt in favor of Vista.
So, what to do? The advice I'd give anyone is as follows: if you have a copy of XP laying around that you can legally install on your Mac, then by all means do it. There is nothing you NEED in Vista right now, and you'll benefit from the maturity of XP. However, if you need to buy a license anyway, the decision becomes more difficult. Vista IS the future--like it or not, Microsoft drives the Windows world (and therefore most of the computing world), and Vista will become the dominant desktop platform over the next few years. Although I originally intended to install XP on my Mac, when it came time to shell out a few hundred bucks for a license, I reconsidered and decided to look forward instead of backwards. No doubt I'll have to put up with the growing pains that Vista will bring, but so far I'm glad that I've taken the plunge.