I know this may be too late for yourself as your post was rather old. However, I figured I would post this for anyone else who happens across the thread.
I had a crystal jacket case and believe it to be the worst case I have ever bought for an iPod family product and I have at least 6 other ipods. The case wasn't snug enough and allowed my phone to slide around inside which caused the back of the iphone to get scratched up. I know now they sell it with a "shield" that covers the back but originally they didnt. In the mean time, my iPhone has 4 scratches in the back of the case which were caused by the case that was supposed to protect it.
I do have like the film which I left on there but as soon as I realized what the case was doing, I removed the case and havent looked back.
For what it is worth, I decided for a 1 month period I would try without a case and its now going on 6 months without a scratch. I am careful what I put it near and keep it in a pocket for just the phone but its been just fine. Just be careful if you have butterfingers.