I will add to the chorus of fixed gear proponents. It's definitely the way to go. But on the other hand, a lot of commuters around here ride the crappiest mountain bikes and comfort bikes possible in order to discourage theft (at least I'm assuming that's why, maybe they are just broke) and of course, better components need less maintenance.
As far as Shimano Nexus goes... YUCK!!!!! I work in a bike shop and have seen several generator systems quit functioning and no one could figure out why. The wiring is really crappy - Shimano should hang its corporate head in shame. The internal geared rear hub makes getting the rear wheel off a pain in the butt. A 1x9 system with a Shimano XT rear derailleur or SRAM X9 would be a much better option - lighter and more efficient. After all, the front derailleur is actually more problematic than the rear.