Phone #1: my fault, I cracked the screen. Oddly though, I noticed the phone audio quality was kinda degraded too...
Phone #2: first replacement. It was plagued with phantom touches at the upper-right corner of the screen (portrait mode), which would start about 10-30 seconds after unlocking it. Very frustrating.
Phone #3: this one seems to be full of dust in the upper-right corner of the screen (location coincidence) - enough that it's difficult to see the battery indicator and clock when not in perfect light, and enough that the proximity sensor doesn't function. A broken proximity sensor means that when you're on the phone and you want to type on the keypad, or perhaps browse your contacts, you can't, because the screen won't go on while the phone thinks you've still got it up to your face.
Phone #4: just ordered this, it's due tomorrow. We'll see if they get it right this time.