I have been using Ogg Vorbis files for ages as my ripped-audio-format-of-choice.
In October 2005 I bought a Hypersound Car, which, to put it mildly, was terrible. I have been searching around for a replacement and the Silvercrest KH2380 might be it. At £50 it's worth a look, and I'll post a review if I manage to find one.
I also have a Samsung YP-U1 and it's pretty good.