Personally, I'd only do it if I knew that a major problem I was having under win2k was only fixed under XP, or some program I needed to run would run under XP but not win2k - other than that, why bother? Upgrade installations suck, as has been noted, and a clean install means reinstalling all your software and moving all your data files over.
Unless there's a compelling reason, I generally don't bother. And for me, eye candy is not particularly compelling ;)
I swear if they pull that one, Im porting all my web apps over to PHP and running linux. Im already POed about their new pricing structure for sql server 2000 (nearly 6-8 g's for a dual processor running web clients!!!!). MySQL here i come!