Pro's: Very light (easy to forget you're wearing it). Usually very good at automatically being detected by my phone (a Red RAZR, which I hate). Very easy to use - one touch to redial. Excellent voice dialing capability.

Con's: Expensive. If you put it in a pocket or bag, the power/call button is guaranteed to be hit and you'll be dialing someone accidentally every time. Impossible to use outdoors -- the slightest breeze makes it impossible for the other person to hear you. There's no warning that the battery is getting low - you have to wait for your caller to start telling you they can't hear you (you can still hear them just fine). Not always reliable.