I've just installed the MotorMouth II in my girlfriend's car. So far we're very, very happy with the unit.
It seems to shut off automatically when she leaves the car, and when first getting in the car, one click on the button will quickly get it reconnected to the iPhone. One more click will quickly answer a call, and another click to hang up. (A nice surprise: Two clicks to answer a second call when already in a call, and two clicks swap calls back and forth.)
She'll keep it mounted all the time, plugged directly into the aux-in at the center of the dash, with charging cable always plugged in as well. The car's charging port is a few inches below the aux-in, so it's a very clean mount.
The noise cancellation seems to work very very well. When I'm talking on my own iphone to my friend, (she's the one using the MotorMouth II with HER iphone), she hears me perfectly well through her car speakers, with no feedback whatsoever, and I hear her perfectly through my iphone. At my end (using my iphone as a standard headset, no device attached) her phone sounds as though she's not even using a speaker phone. And this is despite the the fact that with the MotorMouth II mounted to the center of her dash, the Mic placement is far from what would be considered ideal.
Audio quality through Bluetooth is excellent as well. She's been using it with Pandora and is very happy.
The only feature I'd love to see added is a button sequence to begin playing audio, so the phone would never have to leave her purse. This might be accomplished with iphone voice command (which IS accessible directly from the MotorMouth II). We need to play some more, and I haven't yet found an iphone voice command for launching and playing Pandora. Still looking.
UPDATE:
I have discovered that the iPhone's Voice command does work with the MotorMouth II. You click the MotorMouth multifunction button once when its in standby, and you can dial by voice, or play or pause music, using commands such as "Call John Smith", "Play", "Pause", or "Play SongName"
However, I still haven't found a way to start playing Pandora without putting my hand on the phone. If Pandora was the last music app you were listening to, and you left it on Pause, when you say Play, it begins playing the iPod app. Despite the fact the the music control buttons in the MultiTasking bar show the Pandora icon (when its in that pause state).
Pete