I had one of these on the DS Lite my kid had. It did work. As stated it was bulky for smaller hands. But it never did break. Even when dropped. I think the situation above is cause it was already closed that it popped open when dropped, then slammed shut again causing the screen to break. Then again I could be wrong. My 6 year old didnt break her DS with the Nerf skin on it and I didnt notice any scratches on it either. It was just old and the R and L buttons stopped working. We got her a new DS Lite after that one broke and she hasnt needed it since. She now has a working DS Lite and 2DS. The 2DS doesnt even have a case and she is able to take care of it.
Greasy hands you wont get around with the Nerf case. That is something that would require shrink wrap.
But the Nerf will protect the outside. It just snaps on. I can see where people would say it causes scratches as it is a solid piece of plastic covering the unit where the Nerf is glued to. So it could happen.