I picked up a Jabra Crusier as I couldn't find a nice way of modifying my HU/Amp/Speakers whilst keeping the car looking decent.
It works extremely well and i have no problems with its frequency getting dominated by other FM sources in any big city (London, Birmingham, Manchester), when you do here a bit of interference you press and hold the tune down button and it scans all frequencies for the next signal with no interference, it always works fine.
I use it for my phone so I can use my phones MP3 player, Sat Nav and of course normal phone functions. It can attach up to two bluetooth devices at once as well and is compatible with phone phonebooks and all the other phone text things that get transmitted (i can't remember all the acronyms).
The only issue I have with it though is that it is Bluetooth (which is what I wanted), but Bluetooth trims the music so it never sounds as great as it does via a CD or MP3 player onnected via an Aux port. So sound quality isn't the best, but it is no where near bad... merely frustrating it is not perfect.
I did notice that FM transmitters work different in different cars though, mine is perfect in my car as well as in my brothers and a couple of my friends but in my dads and a couple other friends it tends to not be as clear (I think due to the way some cars are built, some sort of Faraday cage effect - possibly). So that is another thing to bear in mind.