I would be careful with how much you leave in the lockers - our stuff was stolen during our last trip. And the door was locked, we checked it several times before we left it.
I'm not sure how often that happens, I'd like (hope) to believe that it's a rare occurance, but it did happen to us, and it is a day-wrecking thing to have happen. Someone had already spent $500 on my friend's credit card by the time we'd called to cancel them.
The good news is the backpack wound up being dropped off at Lost & Found and we got it back on the way out of the park. Amazingly, they didn't take any of the cash, but they did take the credit card and the digital camera, which had all of the pictures of my friends kids' at WDW. We never found the camera.
So if you have to use them...watch out what you put in there. This upcoming trip will be my first time back since that whole thing, and I'm already working out how little I'm going to carry so I can avoid the lockers if I can.