the differences between the 740 and 1640 is that plextor makes its own firmware for the 740 and the benq makes their own for the 1640 so they each can only use their own software and theres probably differences in the write strategies. I heard you can cross flash 1640's to 740s and vice versa, but im not totally sure.
the difference between the benq 1620/25 and the 1640 is the 1640 has faster dual layer and +/-RW write speeds. The 1640 also supports dual layer -R media whereas the 1620/25 dont. The benq 1625 just has lightscrube, its pretty much the same as the 1620 AFAIK.
IMO, its not true the 716 is better than the 740 or 1640.