I personally like XP, its code is as stable as Win2k and its a gaming system. You get NT stability and the NTFS file system security as well as a gaming system. Win2k is extremely stable but is not to gamer friendly...XP offers full gaming with the stability, besides the XP registration on only 1 system per copy limitation

I haven't found a downside of the OS yet.
You have to be careful of which version of XP you buy, basically XP Home is domain crippled which means it wont interface smoothly with an NT domain it will connect but cannot be administered and you have to keep entering passwords and usernames... its a pain, and really isn't meant for anything other than Peer to Peer networking (max 5 machines)
XP Pro has full network features enabled as well as encryption and some other toys, but if you don't have a home network with an NT domain and don't plan on ever having one save your money and buy XP Home.