I recently added a NEC 3500 to my system, in addition to my BenQ 1620.
I had about 25 Prodisc S03 discs lying around that I hadn't bothered to use up because the 1620 would only burn them at 4X. This equates to a burn time of about 16 minutes, followed by a verify time of about 9 minutes totalling just over 25 minutes.
With the NEC 3500, I've been using Quikee's hacked firmware http://freeweb.siol.net/tvajng/Quikee_FW.html, and burning my 4X -R Prodisc S03 discs successfully at 12X. The end of the burn is a little bit dirty, but not enough to matter. The burn completes in about 6:45 and then the verify adds an additional 5:00. So I get a complete burn that's fully verified in just 11:45. That gives me plenty of time to do a CDSpeed transfer rate test if I want to. The transfer rate test takes an additional 5:00 and straight lines up to a maximum of 16X when done on the NEC, indicating a good burn. Or if I want more detailed info, I just pop the disc in the 1620 drive and use CDSpeed to quality scan it.
I think the BenQ 1620 burns a little bit cleaner than the NEC, but so far all of my NEC 3500 burns are working really well.
I keep hoping that someone will successfully hack the 1620 to give us 16X read capabilities, and allow a successful overspeed for some of our media. But so far this has not happened.
Over the last couple of weeks I've used up all 25 of my remaining Prodisc S03 discs and the ones I've tested have all played fine in my Philips 642 set top player. Next up, I've got about 40 HP CMC Mag F01 discs lying around, and it's looking like the NEC 3500 can successfully burn these at 12X as well.
As far as reading is concerned, a movie rips in about 6 - 12 minutes on the NEC 3500. So far I've ripped about six movies without any reading issues at all. It's just the opposite - the movies generally rip faster. I suspect that the BenQ probably is more reliable when it comes to reading scratched discs - but the real winner here has to be LiteOn. I have a LiteOn 451 burner that's on another system, and I've seen it read more than one disc that will NOT read back on my 1620.
Over the last three weeks, the NEC 3500 has impressed me far more than I expected. I would strongly recommend keeping one of each.