They're very durable. I've had a number of riders take some pretty insane hits with them and the cameras are fine. Sometimes you'll have to replace parts of the mounts though as those will occasionally break. I've had a tall mount twist completely sideways but the camera stayed on the helmet. Only time I've ever had a camera come off was a couple weeks ago when a mini rider went down. However that was thanks to how the Fly helmets work, they have a ridge that goes down the middle preventing you from getting the full sticky pad on the helmet.
This was also the first time I've ever had damage to a case, it broke part of the crappy Hero 3 locking mechanism (this was fixed on the Hero 3+).
Bought a new Hero 3+ case on Amazon for $30 and now it's better than new.
Also regarding your water question, as long as you use the waterproof backdoor you're good down to 131 feet.
They're also pretty much set and forget, get the settings how you want and then it's as simple as turning on and hitting record.