I had a RollBak installed a month and a half ago. Here's my observations.
Haven't notice any moisture in the bed, but haven't had it in more than a light rain (the drought down here).
Rolls back real easy. Almost too easy when it slips from your fingers and you cringe when it hits the box. No damage the few times this has happened.
The attached cord for closing it from the tailgate area constantly gets in the way or tangled up with what's in the bed. I'm real tempted to put a hook near the top of the bed by the tailgate for it to see if that might help.
The cover latches to the tailgate so you can not open the tailgate without at least partially retracting the cover. Now picture that in a rain.
Locking/unlocking two locks (cover & tailgate) is a slight pain.
There is a slight visible shiver in the cover at highway speed. No noise.
Buck antlers can scratch the box. The plus side is when you pack a deer with ice and close the cover, the deer does stay cool.
Overall I'm pleased with the RollBak. About the only change I'd like to see is to latch to the sides instead of the tailgate.