The Plex px-716a could be a true workhorse IF you get a good drive to start with. If you really want a Plextor, try to find it at a good price from a brick and mortar store--especially if you are in a good-sized or big city and you're already close to one. 716a's have had quality problems that Plextor for the largest part has not corrected, and even during RMAs (when you have to send it back to Plextor for a replacement), Plextor has sent people from 2-6 additional bad units until they finally sent a good one, and at least a few people were repeatedly out major $$$ for shipping those drives back to Plextor. So, if you get one, get one from a reputable computer store with a GOOD return/exchange policy. Best Buy should be fine for that--if they have a good price on the 716a. Otherwise, the BenQ1640 is looking very good on quality burns and has a better cooling system than the 1620, but is still not available in the US. (You could, however, buy a Plextor Px-740a and crossflash it to a BenQ1640, as the 740 is actually a BenQ1640 with just the Plextor tag on it; however, crossflashing would VOID your warranty, so if it died, you'd 'eat' that drive). The NEC3520 is also a good drive and has very good burn scans to its credit and has a lot of satisfied users.
Check these out in their respective burner forums and see what you think. It is possible once the BenQ1640 comes out in the States that you could basically buy 2 drives for the price of one Plextor 716a (unless they really drop the price on it), which would let you 'play' with two drives for the price of one. HOWEVER, don't rush to a decision; be patient, research it a little more, THEN make up your own mind. Also, check out the Bargain Basement here from time to time to check on deals, for burner deals and DVD media deals come up there often.