I just got a BenQ 1640, and so far quality tests are very good. I have put it in my main machine and am putting the BenQ 1620 in my secondary machine.
I was kind of nudged towards the 1620 sometime back based on burn quality and with only a hiccup or two trying to burn Verbatim 8x at 12x (silly me), it has performed admirably.
I'm using Verbatim 16x DVD-R media with both drives and all my burns with them have been at the full 16x without any readability or movie playback problems at all, thankfully. All 4 spindles I got at Best Buy show to be the same media:
Unique Disc Identifier : [DVD-R:MCC 03RG20 ]
Disc & Book Type : [DVD-R] - [DVD-R]
Manufacturer Name : [Mitsubishi Chemical Corp.]
Manufacturer ID : [MCC 03RG20 ]
Blank Disc Capacity : [2,298,496 Sectors = 4.71 GB (4.38 GiB)]
So, I guess I'd recommend the BenQ 1640.
Good luck on your purchase.