I realize I'm not making any friends with this statement, but I continue to be baffled and amused by the popularity of fixed-gear bikes in this city.  We have 
hills, people.  We have hills so steep that the sidewalks are 
staircases.
Invariably, an SF fixie rider:
- is about 20 years old
- isn't wearing a helmet
- has several tats, piercings, etc.
- dresses so as to display them as prominently as possible
 
All of this feeds my impression that it's all just a fashion statement.  Riding a fixie around this town is simply nuts.  Sudden stops (kinda important in city riding) are nigh-impossible.  The steep pitch of all the climbs and drops means these kids are going to be paying the orthopedic surgeon for brand-new knees before they turn 40.
Just now, I saw something that put it over the top for me: a fixed-gear MOUNTAIN BIKE.  Yep.  No brakes, no derailleurs, big fat knobby 26-inch tires.  But gosh, what a sweet paint job, with rims and tires in matching tints even!  And me without my camera.
This, kids, is like going snowboarding in stiletto heels and a monokini.  Painfully stupid, pointlessly dangerous, borderline life-threatening... but damn, it looks HAWT.