Disappointing that it is not very compatible with the 3gs with the most recent iOS. The main button doesn't function at all. I could not place calls through Siri using this device and I could not end phone calls with the button. I could not change songs with the device. I did not try receiving calls but I am sure I would be able to answer them either.
The device did not turn off when I turned off the car and took the keys out. I guess that has to do with the car, but this was also very annoying.
On the upside is that the voice from the other end of the call comes through the stereo very clearly and so does the music from the iPhone.
I was doubtful about this but it works very well. Imagine that the big button with the frequency reader is like the button on your apple headset that comes with your iPhone. You can play/pause music, double click to skip to the next song, hold it and ask Siri. I'm putting 4 stars because I haven't tested the hands free kit yet with an actual phone call but it is very promising so far. Siri understood but the sound of the voice was a bit low through the car system. Very easy to set up and nicely done. It feels a bit plasticky but it's so simple to use that you will forgive this.