If it's as good (or bad) as the iMac thingy, it's passable but not great. Why not just get a head unit that has an aux input in the front and plug your mp3 player in that way?

Or hell you could just get a MP3 playing head deck now for about $100, like this one (which probably sucks but not as bad as an FM transmitter!) http://www.mp3playerstore.com/buy_it_now__/SL-5MPVX.htm