For starters, my experience has been with the built in FM transmitter on various Sirius radios They work great if you stay in one area. However, for long road trips, it seems as if every 50-75 miles you are changing the stations - a giant pain. Naturally, a hard wired FM transmitter works MUCH better. However, given the price, my thought is why not spend a bit more and get something that is a direct connection into the radio.
My second complaint is that the FM frequency is limited to the frequency response range of 50 to 15,000Hz. Sorce:
http://www.cybercollege.com/frtv/frtv017.htm
Yes, of course Sirius and MP3 players are a compressed source to begin with, but using higher bitrate MP3's will make the limited frequency response noticeable.