You're going to get a *LOT* of opinions on that very loaded question. James and Webslinger are going to adamantly be against using Vista. I use it myself, but, I don't always recommend that others do so. I'm a sucker when it comes to pain.
My experience with Vista on my HTPC has been mostly positive. Other people see absolutely no reason to leave XP; especially on an HTPC. The thing with Vista is that it comes down to drivers as to how well it's going to work or not. And the state of drivers in Vista, to put it very bluntly, SUCKS. If you get hardware that you KNOW has good Vista support, then Vista will be an insanely stable foundation upon which to build on. If you have drivers that quite frankly aren't good, then stability is going to suffer and you'll be tracking down problems and not knowing which way is up. If you do decide to go Vista, there are plenty of us with experience that can help, but, there are just as many that are willing to let your ship sink.