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I've had mine since they were released and as for manufacturing defects nothing to report here. I don't even have a case for it and I've dropped it twice (not smart I know) and it still seems good as new. The back has gotten scratched up but if I cared I would have bought a case.

I think in terms of the camera (unnecessarily sucky) and iOS (too restricted) it is kind of disappointing as a device but if you are only worried about build quality I have no complaints.
 
I bought an iPod Touch 4G 32GB on the Black Friday sale from Amazon. Arrived quickly, no hardware issues so far. It was a little confusing on first setup since I was replacing an 2G Touch, but once I figured it out no problems. It's definitely in good shape - no light leaks, no dead pixels, sound is fine, buttons all work.
 
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