I had three replaced under Applecare. None had any outward signs of damage-all had wires break inside the insulation just past the power brick despite the fact that I was always careful to use the "official" winding method of leaving a large loop sticking out from the first turn. Most would work if the wire was held "just right", but were otherwise unusable.
All of mine were replaced no questions asked when I took them into the genius bar. They just verified that my old charger no longer worked, then brought a new one out from the back and confirmed that it worked. Every time, I think I was in and out in 15 minutes or less.
Knowing that my Apple care will soon be running out, I've since been stocking up on spare(OEM) chargers on Ebay. Rather than carrying one with me all the time, I just keep them at a couple of locations at home and keep one at work. This saves wear and tear as I just have to grab the cable and plug it in. I'll also add that I generally buy the T-type chargers, which I prefer to the current L-shaped ones.
