I've had Seido cases for a 7100 and 8700 - never noticed scratching, but the new rubberized case for the 8830 is really soft where it touches the phone - sure, if there's a grain of sand or something sticking to the rubber you may have problems, but otherwise...
I am noticing a tendency for the top clip to want to "click" in the speaker slot - I manually open the clip & hold it open while I put the phone in. Then again, I'm still in the "honeymoon" phase and am being overly gentle
-mj