I recently switched from the Apple to Tv to a Mac Mini as an HTPC. Prior to the Apple Tv i had tried several proprietary single box solutions; anyone remember Turtle Beach or Roku.
The single box propriotry solutions, and i would lump Boxee into this were for lack of a better word disasters. SOftware that had not been properly tested, hardware incompatibility issues and limited playback formats. I suspect that the WDTV would suffer from the same limitations. To much competition in the market place. i give Western digital 12 months before they drop all development and support of the UI
My suggestion would be to go with the Mac Mini run with a USB to and external DAC; the Benchmark is a slam dunk in my opinion.
For the UI load Plex currently in pre-release but its interface is gorgeous, plays back video and referecnes your itunes library