I received mine and I have some bad news. it certainly does affect the signal strength on my 4S. this didn't surprise me no thanks to the crappy AT&T signal in my area. my signal has always been bad but now I can barely use my 4S with the Ringke case.
Ditto on my 4. I ordered the slim case but they sent me the steel bumper so I gave it a try to test reception (reason why I didn't choose this case). Lose about a bar on average.