I have both the Pioneer DVR-109 and the NEC 3520. Out of those I personally prefer the 3520. Mostly due to third party firmware which is more mature imo and supports such features as bitsetting and 'overspeeding' (burning, for example, 4x rated media at 12x). The Pioneer wants to burn at the rated speed all the time. It is also much harder to use bitsetting on the Pioneer, but still possible if you crossflash to the 'Buffalo' firmware. Using DVDinfopro to scan results, I also get better burns from the NEC on cheaper media. On high end media both do a good job.
If price is no object, the Plextor 716 model is a good choice. It has a lot of little features and software to play with. They cost almost 3x as much as other drives and IMO the price is not worth it, although I do think they are a very good drive. Beware of the SATA version, that has a lot of compatibility problems at this point. Check the Plextor website and make sure your motherboard is compatible before buying.