It's a little radio transmitter which takes its power from your iPod or iPhone (I'm using it with my iPhone 3G and iPod 5G). Bonus is there's a (shortish) USB cable and plug for your cigar lighter socket (I have an oldish car), so that you don't have to drain your player battery. Plugging it in doesn't seem to help the signal strength.
So you set its transmission frequency and tune in your radio (I had to turn off the auto frequency control). You then get a reasonable signal, especially if you have followed the recommendation to set the frequency to somewhere there's static. Obviously if you're travelling a distance, you may need to reset the frequency (since what was static in Stoke may be noisy in Nottingham) or suffer deterioration or loss of your puny signal.
I get a reasonable reception with the iPod/transmitter on the dashboard (using one of those anti-slip mats which I've reviewed elsewhere) and my aerial in the rear screen (it seems to be integrated with the heated rear window elements). It might be better if I put it on the back shelf, I suppose: I've not tried it.
It is significantly poorer than an aux input, but gives acceptable quality.
So: a good buy, better that a big-name product I tried at three times the price. Just don't expect miracles.