I have the JVC SH-99 and love it. It is a very solid head unit, offers great expandability, and also offers some tuning via an in-unit equalizer. As far as MP3 playing, it is much faster than my old Aiwa MP3 at reading MP3 files and also has not skipped since I bought it
Other than the JVC, I would recommend Clarion's new MP3 unit, I have not personally tried it but I am a huge fan of Clarion and their products. Prior to the JVC and Aiwa, I owned a Clarion head unit and have nothing but praise for it. Sound was clean, expansion capabilities were great, and the ability to add an equalizer unit that worked through the head unit was great.
Between the Clarion and the JVC, I would recommend the Clarion because of their long-standing reputation. If you are just an average audiophile(like me), the choice will probably depend on which one you find to be more aesthetically pleasing because their specs are virtually deadlocked.
I would have bought the Clarion but it was not available at the time that I purchased my JVC. I'm happy though, the JVC has more than met my needs and has performed flawlessly so far(knock on wood)
If you are looking for more in-depth about specs and the like, you'll just have to wait for Dave or someone to help you out there, I just don't feel comfortable talking about things that I don't completely grasp
