I used a laptop USB port on my RPI and blew out the power light or I guess as it was later decided one of the resisters going to it. 2 days old. While you can use a USB port on a computer to power it, according to what I have been told by the Raspberri Pi group is that it is very highly recommended to use a Cell phone charger with at least a 1A output because anyhting lower will cause issues with the unit.
I am waiting for a new Raspberry Pi now since I sent the other one back, but hope to be in business soon with it again. When it comes to XMBC, you will find it is a tad slow on this unit (I had the same one), so don't expect it to be super fast. But it does work good for everything else with the XMBC software.

Also, video playback on some channels like HGTV or Food Network don't work as they haven't been able to license the video decoders for those networks.