The only problem I've found with the FM transmitters that go in the car is that they're often easily blocked out by genuine radio stations on the same frequency. Whilst I have one that works perfectly fine in my local area if I drive south 10-15 miles it starts getting blocked by the local radio station in that area. Tad annoying when you're trying to listen to your music and you can make out crackly voices/adverts coming over the radio in the background.
Before you purchase one check to see what frequency the radio stations in the your area are operating on, no point getting it and realising that it's of no use to you.
Edit: My friend has this one:
http://www.amazon.co.uk/TeckNet-MT-0...d_sim_sbs_ce_2 You can adjust it from FM 87.6 MHz - 107.9 MHz meaning if you're getting any interference you can hop to a more vacant part of the spectrum.