well, do you want a portable one and an interface with your car's HU or do you just want a HU that can read MP3's that are burned onto a CD?? and if you want to go the portable route, what do you already have in your car so we'll know whats the best way to link the two??
I have the Kenwood MP-928, I paid around $300 shipped on eBay, got a great deal. I like the HU a lot. The graphics on the face compare with the best I've seen anywhere. The software that comes with it for burning CDs is excellent. It has voice prompts that say what band/album you're currently listening to. It also has options to display the cover art of albums on the HU. Probably the best feature is that it loads tracks quickly.
I had a Pioneer MP7400 in my old car, and I loved it. The only problem I had with it, was that it took a noticeable time (3-4 seconds) to load each song even if it was just playing them in order. The Kenwood loads virtually instantly, just like a normal CD.
I know a lot of people complain about Kenwood's here, but I've been very happy with my HU so far. I disagree with people that say they don't sound good, as it sounds very comparable to my old Pioneer. It just took a bit of tinkering with EQ settings, as there's no easy presets that sound any good.