I always go for the good old Sony Ericsson bluetooth kits. They're anything from £15 to £35 on eBay normally. They take around 20 minutes to fit to your car once you remove the stereo and they're fantastic. They merge with your stereo and work through the car's sound system.
I find they often work better than the Parrot kits at a fraction of the price. The only downside is that they don't do caller display, but for the price you can't go wrong.