Apart from Boxee, AppleTV and very recently perhaps the new PopBox, there isn't a device out there with a usable UI. That means method of navigation, what it shows you when you navigate, how easy it is to find your media, etc. As a whole, the UIs look terrible, don't offer you any usable information besides maybe a filename, including extension, and have really really (REALLY) bad performance.

The WD box has some attention to the UI, but it's still really just about files and doesn't know anything about the media nor metadata. Apart from the devices I've just mentioned, no one else is at all concerned about UI nor what gets displayed on the screen apart from perhaps being able to decode the video files. Even simply pausing or skipping around though a file is a painful experience on most boxes. Unfortunately Boxee still doesn't have this right either, since in their current beta it still freezes up the video if you skip more than twice in succession. The BEST for this is SageTV, bar none. Three sets of forward skip, three sets of backward skip. You can change the amount of seconds/time for any of them.