I use this:
Newegg.com - Motorola Bluetooth In-Car Speakerphone Handsfree Car Kit with Built In Digital FM Transmitter / Audio Caller ID / 18 hours talk time (T505)
Sound is decent, you can have it play the other caller over your FM, and it turns on fast enough I leave it off until I hear my phone ring. It's not perfect but its decent enough for the price.