I have one and i haven't noticed a functional difference but if you go to the signal of the phone with and without a metal back, the signal number is definitely higher (which is more negative and worse). My husband has an s3 and i hold mine and his in the same place and mine has a metal back, mine is -100 and his is -92. when i put the stock battery cover back, it's -92 as well.
i like how it looks much better so I'm debating. like I said I haven't noticed a functional difference (dropping calls, etc) but I did want you guys to be aware...