The purchases thread has been the home for this, but I thought a seperate thread would be nice.
I've got mine all set up now, and it's awesome.
Installed raspmc RC4 on it, and plugged in a 1TB external hard disk filled with TV and films. Have been happily media centring away ever since. Plays everything nicely, including some 720p stuff. Havn't tried anything at 1080p. I've also configured a Samba server (so I can mount the external drive as a windows file share on my housemates computers so they can add stuff) and made a few changes to the remote configuration (you can use a TV remote to control the Pi, but some buttons needed remapping).
All in all, pretty awesome. It works really really nicely. Definitely consider getting one for media-centre type stuff if you're cheap and have/are not adverse to learning some basic Linux skills.