If you want to get a FM transmitter spend a few dollars more and get one with digital tuning which gives you the entire range of frequencies. If you get those with only 4 or 8 preset channels then most likely you will find that all the available channels are already occupied or too close to commercial broadcast so it will not work.
I first got a 4 channel model, returned and got an 8 channel model, returned again and finally settled on the Belkin one which has a digital tuner. The digital tuning ones also will have memory preset channels so that you can switch between frequencies quickly. This is useful when you travel one place to another with different commerial stations in the airway.
If you want to use it in the car, get one that has car power adapter so that you don't have to worry about batteries.
Sound quality is comparable to FM broadcast. Take some time to find the optimum volume setting on your MP3 player. If your MP3 player output is too low then you have to turn up the volume o your car tuner and there will be more noise. If the MP3 output is too high then it might overload the transmitter and you get distortion especially when there is heavy bass in the music.
I kind of like those which has the FM transmitter built-in to the car adapter. Very sleek idea but I am not sure if they have proper filtering to prevent the noise fro the car electrical system from interferring with the sound.
Happy hunting!