Yeah I was wondering this too... is it like the GPS sensor where its not really doing anything until its being used?

 

even while on it constantly scans for devices its paired to, i recommend turning it off while not in use

 

This.

I was leaving mine on and it does indeed chew up the batter. I've been turning it off now for about a week or so, and have seen improved battery life. I just fire it up when I'm in my car.

 

I personally will not shut any features of my phone off because I feel it defeats the purpose of having something like this. However, after several days of watching how much juice BT eats up while inactive, I now turn that off when not in the car. It's really a much bigger hog than you'd think. As much as it pains me, I added the BT widget to my home screen, but it really is a pain in the neck to remember to turn it on when I get in the car. I'll be picking up a larger battery soon, maybe then I can stop worrying about it.