Fantastic.
Never before have I had such a strong, quick bluetooth signal with any combination of headset and phone.
This same headset would connect slowly to the iPhone 3GS, or sometimes make a lot of static and then disconnect.
Not with the iPhone 4. I talked for almost two hours to various people. The quality was very clear. When someone called, a touch resulted in a picked up call near instantly.
After turning the Jawbone on, it quickly connected to the phone. It was ready for use with voice dialing almost immediately.
Inside the house I was able to go 25′ away with no audio quality loss.
I am very happy with the iPhone 4’s bluetooth prowess!