I have the same problem. Nero6 Express failed verification for 4 Verbatim 16x dvds (one was written in 4x, rest 16x) and Ritek 4x failed too, as well Commodore 4x dvd. Some of the files work on the dvds but most are unreadable and ScanDisk in Nero CD-DVD Speed gives lots of red (bad) sectors, 99% for most disks.
I'm returning my LG GSA-H20LB today. Maybe I'll take GSA-4167 instead, as it seems to have a good review.