Since there are tons of new people here, I thought I'd post a link to this comparative table looking at the Riocar (empeg Mk2a), PhatNoise's Phatbox and the SSI Neo. I've updated it to reflect the recently added support for WMA on the empegcar as well as a few other minor changes for the other players.

Direct clickable link: (auto)mobile Jukebox matrix
Plain copyable link: http://www.mypocket.com/mobilematrix/index.php

If anyone has any corrections or ideas for additional criteria, let me know. There's at least one new player on the market which would make an interesting addition. The recent Sony 10GB and Pioneer units might also make it into a future revision (though I'm leaning to disqualifying anything that doesn't support MP3).

Bruno