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I JUST got a week 28 in the mail. ;) I haven't opened it up yet because I'm at work, but I'll update as to any issues with it tonight.
 
Build Quality is worker dependent...and that likely does not change week to week...but rather on a shift basis...ie 1st shift, 2nd shift, etc. The thing that matters the most I would bet is the components that get used...some weeks may have a bad batch of LCDs resulting in a warmer screen and uneven coloring...or the other components may not fit together as well. That COULD change on a weekly basis.

In the end it's all a guessing game and there are likely good ones in every batch just like there are likely extremely bad ones in every batch. I really wouldn't pay much attention to build week...just get one that works for you.
 
I haven't seen any other weeks so I can't say for certain but my week 20 is perfect. It has a beautiful screen with no discoloration or dead pixels. The bezel is great and no light leaks.
 
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