No case has been fine for me, for the most part.
I got the 4s at launch. About two months later, I accidentally knocked it off my kitchen counter. It fell to the hard tile floor, and the hard tile floor won decisively.
Took it to the Apple Store, said it was completely my fault and asked if there was anything they could do to fix it. The rep looked at it and said no. Then he asked if he had been giving me any trouble. I said no, that it worked flawlessly. He said he wanted to run a diagnostic to make sure. He ran the diagnostic and told me it had spit out an error message and thus they needed to replace it at no charge.
I thanked him profusely. He went to the back to get the new phone for me. Then he grabbed a case and said "I HIGHLY recommend you get this too." Couldn't really argue since he hooked me up. I never did use the case though.