I previously owned a Griffin Roadtrip FM transmitter, which I was pretty pleased with when I had an iPhone 3GS. However, when I got an iPhone 4S the Griffin could no longer hold it (no suitable cradle). So a replacement was needed...
This product was substantially cheaper than the Roadtrip, so my expectations weren't too high. However, now that I have received the item my first impressions are good. Set up was a doddle and I didn't really need to read the instructions. It found a good signal quickly and locked in well. Better than the Griffin unit actually. Also it seemed to hold the reception at least as well as the Griffin and probably a bit better. the fact you can also get an accompanying free iphone app that makes channel selection/changing even easier is a plus.
A good product so far - I'd recommend it, as long as you are aware of the general limitations with FM transmitters and have sensible expectations of the sound quality (bit like listening to the radio).
One thing to be aware of - this almost didn't fit my Ford Focus. In the Focus the cigarette lighter in sunk in a deep recessed 'cubby hole' - this made it hard for the 'arm' of the device to fit in...i was able to manage it, but i can pretty much just put it in one positon, which I'm lucky happens to be OK for me. Bit of a struggle though. I think this is the problem with other fixed arm FM transmitters too - a lot depends on the placement of your cigarette lighter/power outlet.