Theres a review of the xbox mediaplayer on the website homeautomation uk. I havent yet read it because i'm on a bus and i'm only aware of the review due to avant-go non my pocketpc.
The homeautomation uk website
The direct review link
The review is a good one but only at the basic level, comparing it to windows xp media centre is a difficult one because of the lack of video input on the xbox.
Give it time though because there are projects on the go in this area.
Project Dreamix is the one i'm keeping my eyes on.