Just got one, and it's freaking cool :cool:
Basically a little box which has HDMI/SPDIF/network and lets you stream high definition video over your home network and play it on your TV.
Supports lots of popular formats, such .mkv files (high def video container) and supports Dolby Digital/DTS sound tracks.
Can optionally fit it with a SATA hard disk and keep the media files there, or play them over the network via CAT5 or using a WiFi USB stick. If you do install a hard disk, it also has built in bittorrent client (which i've not tried yet) to download your stuff (legally of course) using RSS feeds.
Only had it a few hours and I think it's great. Really portable, has an external PSU which isn't too big so you could take it to a mates house and play your warez