No experience with the others but I've been running MCE since last December. I like it and you can use it for serving your music, pictures, surfing the web, listening to subscribed music, NPR archives, weather and so on. There are other software choices too but at the time I looked at it MCE was the only one supporting HDTV. For now it's only OTA HDTV, but I think it will support certain cable boxes with FireWire out later this year. The nice thing about a Windows based solution is you can run other programs, upgrade the hard drive and add/change components. The XBOX 360 will have Media Center Extender software built in and it will support HD.

OT: Here's an interesting article on 1080p for those considering it. My current set only does 1080i.

The Clicker: Futureproofing with 1080p? - Engadget - www.engadget.com
http://www.engadget.com/entry/1234000303057041/