MP3 play capability is nice indeed. I no longer feel the need for a CD changer. 1 CD can hold so much.
Being rather frugal, I am content with my low-end Sony MP-40 for < $200. Mostly black. Some of the buttons are a bit smaller than I would like.
The very nice Alpine CDA-9815 mentioned and others will play WMA files, which have even tighter compression that MP3.
These things become obsolete so fast, it is hard for me to invest much in car audio. If I bought a high-end unit 2 years ago, I would be cursing that it could not read MP3s.
The fellow who mentioned the embedded hard drive is onto the next big thing. I just bought a 140GB hard drive for $99, and a $30 USB enclosure makes it portable. Now that's a lot of MP3s! At least 1400 hours of mid-quality music. Just a minor matter of 'convergence' to make head units capable of 'plug and play' with IDE or perhaps the coming serial ATA devices.