It's not very good to be honest.
It cuts out a lot, the iPhone can't be too far away from your headphones and can't have too many things covering it (i.e. if you put it in a carrying case in your bag, it flips out when it gets a little more than 2-3ft away).
The iPhone gets somewhat hot after about half an hour of A2DP.
Also the lack of AVRCP is a HUGE fault. This is the profile that allows you to control the songs (next, back buttons) from the headset. I'm betting that's because Apple is going to release their own stereo bluetooth headset with special apple controls over a special profile.. I will vomit if they do that.
On a good note, it doesn't drain my battery at all!
A2DP takes VERY little battery power on iPhone 3G! Not sure why.