The battery drain that occurs is very miniscule and it's called "Zombie Drain" You might notice that when you enter the vehicle, some of the lights come on for a bit and other stuff is awake faster than it might be if you rapidly approaced the vehicle and pushed the Start button. I think you would need to leave the fob in the vehicle for at least a couple weeks before you would notice any problems from the zombie drain.

All in all, I like the keyless systems and we got the first one in 2009 and have only had to replace the fob batteries once. I guess we're down to no more keys in any of our vehicles except the RZR. I wouldn't mind a fob for that either.