richms: The inbuilt stuff was very underwhelming when I tried it out. Not sure if it is a limitation of the TV or the upnp media server, but it was slow and annoying to find anything on it in a large music collection. Limited formats too.
I was considering a WD TV live, but have had no feedback on its UI yet.
I don't own a WD TV but I've used them a lot and am planning to get one at some point, it's easily the most complete one out there, if it had wifi and played WMVs it'd be perfect. The UI whilst not very flash looking is very simple and intuitive, even to not so tech savvy people. One main menu with movies, photos, music and settings and then they just open up to the folders where your files are and you play them, couldn't be simpler. You can make it a little more fancy if you want but for most people just 2 or 3 pushes of the select button will get them doing what they want.