I have gotten 2 replacements because of light leak but it was actually because of some scratches in the chrome 😀
2/3 had light leak, with the first originally bought one when it first came out in a nearest Apple Retail Store having the smallest light leak, about 1mm. The replacements have light leak as big as the default lockscreen clock.
2/3 had "clicking" in the recorded mic sound
1/3 didn't have a flush screen, squeaks when I press the screen with a pretty tight rubber case on, don't know if other ones didn't have a flush screen too
So, the first one I bought actually had the least problems! 😕
The screen really attracts smudges. Have to wash them with water every few days, especially when my friends see my iPod touch. The chrome back attracts smudges and dust, so it gets scratched up really easily.