Add the Liteon 1693 to your list. It is better at scanning than BenQ (fewer intervals) and as the 1640 is new it might take a little time for it to "season", although it does not yet appear to have the problems of the BenQ 1620. I just added a 1693 a few months ago and I have been very suprised at the high quality burns. It also supports bitsetting and has very good rip speeds. Lastly, there is huge support here for firmware modification should you ever become interested.
The Media receommendations above are dead on correct. Follow his advice and you will be very satisfied.