Well, the 3500 is probably the best they have made to date, closely followed by the 3520. The 3540 never had a very good reputation and the 3550 was better than the 3540 but I have never compared it to the older models. The 4570 is brand new and all NEC drives have needed many iterations of firmware to reach their best. Unfortunately, my 4570 is junk on otherwise good media (MCC and TY T02). It is only good on TYG02.
I have no idea why NEC has this pattern of bad firmware slowly followed by good. You would think that they would have figured out how to burn TY T02 by now, but apparently not.
If I were you, I would switch to a different brand. I love NEC drives, but they are a very frustrating company. Given that Liteon now owns BenQ, you could look at Liteon, the BenQ 1655, the Pioneer 111, or the LG 4166. All of these have been very good burners for me.
If you really want to burn at 16X, the LG 4166 is the only current burner that does so without problems. All the other burners are better at 12X.