I honestly have not had any issues with my Plextor 708UF at work, so I don't think Plextor has had an extremely iffy track record as some would suggest; rather they've sometimes had issues that took a few firmware updates to work out, mainly concerning media compatibility. That, and other people currently have better internal burners on the market for less (NEC and Pioneer)
If an external is your only way to go, I'd go Plextor. Otherwise I'd choose an internal NEC or Pioneer. USB has yielded mixed results for people, what with a higher CPU usage (I wouldn't multitask while burning, that's for sure) and slow transfer rates. I use firewire with my Plextor, which has worked quite well so far.
Intel Q9450 @3.2GHz, Gigabyte GA EP45-UD3P, 4 x 2GB G.Skill @4-4-4-12
Antec P160SW case (modded), Xigmatek 750w PSU
3x 500GB (RAID-5), , OptiArc 7200S, ASUS E818A3T
Creative X-Fi XtremeGamer, Hauppauge HVR-1800, Radeon 4890