Just wanted to add that I also use a WDTV Live with a USB hard-drive, and it is an excellent piece of kit. You can also copy all your media files from your main PC to the WDTV Live's hard-drive, using its interface (this is what I do). Or you can always stream video from your main PC to the WDTV Live, if you don't want to take the time to copy files to the USB Hard-drive.
It's also compatible with some USB Wireless-N adapters. I use a Linksys adapter (
this one) to connect the WDTV Live to my wireless network. I've had no problems streaming 720P video (mkv's) to it from my PC, or copying videos to it over wireless.