As someone who has been using a bicycle in two different ways for a very long time (Centuries and commuting) I'll provide my own pov:
Is it a hipster fad? Yes, and there's some ridiculous stuff out there: no brakes? Hugely expensive bike bling? But SS/fixed bikes are popular for other reasons. I own two vintage steel bikes. One an old panasonic I bought as a beat up ss/fixed frankenbyke and the other a stripped down Trek road frame I got on the cheap. I built the Trek to be better geared, but it's also a bit large and was bought rebuild/resale and to satisfy my bike mechanic geek sideI rode the panasonic as a commuter. High-geared, flies on the flats and downhill, but hard on my knees uphill, and a little too large for me. I just found a cheap used Bianchi San Jose to replace it for commuting, and am finally getting a used geared bike for some long rides this Summer.
Con: A geared bike will always be better than a fixed gear for most riding. People usually discover this at some point, which is why single-speeds are now more popular than fixed gears, and why internal hubs are growing in popularity. If you can only have one bike, make that bike fit what you need it for. A geared bike might be the best choice.
The urban/hipster fad came from bike messengers, who are seen as outlaws, are (or rather, were) omnipresent in every city, and were very fast proficient riders with simple steel bikes that were ridden hard everyday and replaced with other simple bikes when trashed. It's easier to ride a high gear downhill so long as one is strapped into the pedals. Uphill? These bikes would have a gear that can be very damaging to a weaker rider. (And it's also been damaging to messengers as well.) Couriers learn how to get through hairy traffic without stopping. Not safe, but a fixed gear messenger bike meant one front brake that basically served as an emergency device. This is not a *good* fad to emulate. But riding fixed also demands proficiency, and there's a certain pride in being able to do so -- so long as the fixie isn't hallway or wall art.
Here's some pros:
Simplicity and environmentally conscious living: Bikes were all singlespeeds once, and we all rode one as a kid, too! 24 speeds is basically the result of marketing ridiculous "improvements" to us which don't add too much to our everyday lives, except an extra trip to the bike shop now and then. A steel bike can last a few lifetimes beyond original parts. I think there is something reactive in saying "I need a bicycle to be a bicycle, I need to lock it outside, and I'll ride a beat-up singlespeed, or an inexpensive, salvaged frame with a set of wheels." In NYC, lots of people still ride old vintage geared bikes. The downside is that those who can't do their own maintenance invariably have to spend money to have a shop maintain the bike. A "tuneup" costs $100, and that might be more than the rider paid for a used bike. (Hell, my locks cost more than my bikes!). That same bike can be a simpler and more cost-effective one with a reasonable single gear ratio.
Commuting: In 15 years in DC, and in NYC where I've lived for a decade, I have rarely made more than one gear change on a commute. The 1-2 gear changes might be nice, but it also adds more maintenance work to a hard-ridden bike. (The weight savings is silly and not an issue.) A simple bike = less maintenance, less parts to steal. Internal hubs will soon supplant single-speeds. (I can't imagine doing it in SF, but DC is relatively flat and NYC basically has one long-*** hill: uptown
City riding creates unique needs: road bikes are better city riders than mtbs, but hybrids offer bombproof tires and upright riding. Flat bars and 28c gatorskins on a singlespeed/fixed gear bike makes for a fast, competent bike if the rider is already experienced.
Winter riding: I loved the grip shift flat-bar Trek multitrack hybrid I had in the '90s because I could ride it in thick gloves, but I rarely made gear changes and the gear was always midrange. A 42x16 singlespeed with winter tires is the perfect winter bike. Less parts to clean and maintain, and without the shifting one can't easily use.
Bike joy: anyone who rides eventually experiences the sense of a bike which fits the body perfectly, and a body which is inseparable from the bike. The experience is sheer joy. Fixed gears are tricky because a high gear can damage knees, but a reasonably-geared bike enhances the sense of full integration. It's not necessary to enjoy riding, but it's something that many riders become attached to and really love.



Reply With Quote
I rode the panasonic as a commuter. High-geared, flies on the flats and downhill, but hard on my knees uphill, and a little too large for me. I just found a cheap used Bianchi San Jose to replace it for commuting, and am finally getting a used geared bike for some long rides this Summer. 
