My Samsung S3, which I use a lot and have always taken very good care of, refused to boot. I was shipped a replacement by T-Mobile (at my expense, $20), but the replacement, a clearly, and not lightly, used phone, had a camera lens so badly scratched up that it was impossible to take a photo in focus. The camera is critical to me, it's how I create the pocketwatch photos you see in my Google+ stream here.
T-Mobile agreed to send me a second replacement, which arrived yesterday. I don't know if the second replacement was a working phone, but one thing was immediately obvious; it was not an S3. It was some smaller Samsung model. Completely different...

Here's my original post with an example photo from the first replacement, which I am still using as a phone. I just can't take photos with it...
https://plus.google.com/u/0/+JeffSexton/posts/aCJLVaacmsc
I will say this, it's pretty easy to call T-Mobile. There seems to be little wait, there's a real person to take to, and they don't routinely hang up on you.