vertical flight:
i dunno why they claim 20 hours, maybe they changed the original battery pack that was one 9v battery to the old one that had plenty of aa batteries. no, i don't use re-chargeables, because everything is an efford - think about buying it, finding the place where to buy it, getting into your car, driving to the place, etc.
to be honest with you, i haven't noticed any special annoying noise from the gearbox. there are a lot of annoying noises from a helicopter and this is choosing between two lesser evils. i fly a jetranger, but i've used the headset in r22, enstrom, longranger, and a109. the bose is still good!
everything about a headset is stressing, the pressure, the noise, the weight. finding the headset with the least amount of each is the key. what type of operation do you do? if you only fly a couple of hours a day, maybe a d.c. is good for you (money-wise), but if you are pushing 5-8 hours/day, i would suggest testing the bose.
bose's warranty also works perfectly (two years i think)! i sent the head with fedex from florida after the mic stopped working. they fixed the mic, plus changed a few other things, returning the headset within ten days. how's that for customer support!