Slightly off topic but depending on the car you have and whether you must integrate your ipod with it, I'd say not to bother.

The newer 6 CD stackers support MP3s so I have 6 MP3 CDs loaded in mine = 4Gb of songs and they all work fine through the head unit - make sure you use ID tags in V1.1 so you can view the song/album names in the head unit.

I have to say that personally I think BMW aftermarket options are a rip off. The adaptors for Ipods and phones are ludicrously priced. Spending the best part of $1k on an ipod and interface for a car is madness when you consider I can get better sound quality and identical functionality for less than $6 :-)

If you don't have a stacker though, then I guess you aren't left with much choice :-(