I have a WDTV Live hooked up to my home network and it works great. It can play all of the HD and SD formats I've thrown at it so far and has a great interface.
My only problems with it is that it lacks a gigabit connection (only 10/100mbps) and occasionally it has some trouble accessing the network.
Also if you install b-rad's custom firmware, you can use torrent, nzb, and ftp with the box (and more)