I have a Griffin FM transmitter for my IPOD. Overall, it's ok. Definitely not the best sound quality but it's good enough to listen.
Basically, if you're looking for cost efficient and don't care of sound quality, then get the FM transmitter.
If you're looking for clarity then don't bother with them.
I also have a Belkin FM tranmitter, which is universal for all MP3 players. This works just as good as my Griffin.