I am in the process of comparing the Plextor 716A with the BenQ DW1640 using quality media like recommended Taiyo Yuden and Verbatim, as well as a few odds and ends media. So far, and I'm hardly halfway testing, it appears my drives both perform very well with good media, and equally as abysmal with crappy media (such as some on sale Taiwan Verbatim MCC media that errored out on both drives). Overall, disc quality scans are slightly better with the Plextor. The Plextor is able to overspeed burn media. The BenQ does not overspeed yet to my knowledge, but this may change with firmware updates. Regardless, to me, a slightly faster burn that has more errors, even if only small amounts, is not as important to me as a more precise burn though burned at a slower speed, since better burns are thought to improve longevity of the disc. My own findings point towards a very small improved burn with the Plextor, which I am sure some would not think significant.
The Plextor performed better audio cd burns at any speed compared to the BenQ. I am seeing a general trend with my media for CD data and DVD+R burns as well, but, I have not done enough testing nor closely examined the data yet, so we will see in due time. The BenQ, though, had more burn speed options for my Memorex Ultra Speed CD-RW 24X
Two other things I noticed was that, first, the BenQ burned a better quality disc (fewer errors on Nero Quality Disc scans) when environmental conditions were hot and humid (Maybe that "Air Flow Cooling System" and "Anti-Dust Cooling System" really works. Or maybe it's the placement in my case that gives the edge to the BenQ.). Secondly, a video burned onto DVD+R with a horizontal distortion in the lower portion of the frame seen on playback on my Philips DVP642 was less noticable, sometimes not seen at all, on the disc burned in the Plextor when I rewound and played several times. This seems a little bizarre to me, but maybe the Plextor corrects or manages the "burning" error slightly better than the BenQ so that the Philips has an easier time managing the error itself.
Like I said before, the better burns by the Plextor were not gigantic compared to the BenQ. In fact, they are quite comparable overall. What may tip you to buy one over the other might be special features of one drive over the other (of little significance for my burning needs as the basics seem to be all I require), warranty period, software included, ability to burn media you have or will be able to purchase in the future (A huge factor for some since every drive has its likes and dislikes.), ability to overspeed burns, media read/write speeds, hacked firmware or tools availability, and of course COST. Since both manufacturer's support their drive's with frequent firmware updates, you can be assured of a long lived drive no matter what you get.
So, if you are looking for more control over your burns with an expanded feature set (such as overspeeding, Gigarec, etc), high quality burns, a manufacturer that leads the way in innovation (some would disagree, especially the fact that reaching 16X on DVD burns is not universal), and a great warranty and RMA procedure if you live in Europe, then the Plextor 716A is for you.
If, on the other hand, you do not require all the whistles and bells, still want comparable rock-solid quality burns (at least by my testing so far) and want to spend only 25-40% of the cost of the Plextor, then get the BenQ DW1640.
As to defective/malfunctioning drives - a LOT of griping goes on in the forums, all anecdotal. My current Plextor is my third. The RMA procedure was quick and painless, and, tech support was very good to excellent. I have read threads about BenQ malfunctions as well. Who has more problems is debatable since noone here knows the actual statistics, yet it seems that Plextor prematurely ejaculated with the release of the earlier 716A's (a lot of reports of bad or poorly performing drives compared to other forums), though now they seem to have their ducats in a row. I have no experience with BenQ tech support or RMA'ing yet, but I have only had the drive for a week or so. Time will tell if the BenQ will last longer than the Plextor - historically the Plextors have been tanks. I encourage you to read the other forums on CDFreaks to get a better feel.
Bottom line: If you get a properly working drive, you will be happy with either drive. Just like life, plan for the worst and hope for the best.