I ran it on my main system at home from April 2005 until January 2007 when Vista 64 was released. It was a great OS, although it could be difficult at times to find drivers. I had to ditch my older sound card, for example.
Frankly though, if you're going to buy a retail OS, I'd buy Vista. It will last longer, and you can tweak it to perform very well by disabling features like the search indexer and defender.