Plex seems good so far, but I have barely played with it and I really need a manual to set i up properly, unlike and AppleTV or WDTV.
Main options on WDTV (which we should refer to it as bc the MacRumors search engine can't find words less than 2 or 3? letters such as WD TV) is to decide how to navigate.
Currently, I use the WDTV just for movies/TV shows. My music Library is way too large to navigate on WDTV or AppleTV. Without keyboard input I would spend more time scrolling than listening. Hint Apple (I already have the BT Keyboard)
Glad I have an Airport Express for my music.
When it comes to WDTV movie playback, I'm impressed. Not just the quality (supposedly it has the same decoder chips as in many Blu-Ray Players)
But I shut off the WDTV on Thursday, turned it back on on Sunday, sorted thru my movies, picked the one I was watching on Thursday and it remembered exactly where I left off and asked me if I wanted to resume from that point.
So far, not one single playback issue.
And from what I have read that you can use it with a car adaptor if you have a built in car video system.