The Plantronics 510 is comfortable to wear for hours and doesn't feel like it will fall off your ear.  It mates easily to my Treo 650 and also to my Bluetooth-enable laptop, which makes it possible to use the laptop for VoIP calls.  Reception is good, volume is adequate and the people I talk with say that my voice is clear.  The controls are a bit stiff, as others have said.  But that means they have good tactile feedback -- you know when you've pressed a button.