I have gone through 3 RMAs with BenQ and seen many more reports of the same for the 1640. Currently there is a lot of work going on trying to figure out which COO is the best to not get a bad drive.
No one has yet seen a pattern that is consistent. If one really wants to get a BenQ in spite of all their quality control problems, you should at least get the OEM and save a few bucks. You be less annoyed if you get hit with one of the defective models and have to pay shipping to return it.
Trust me, your chances of a bad drive are equal no matter what you do.