Fixed gear bikes are among the oldest forms of two wheeled travel known to man. If you are looking for a mode of transportation that fits in at dive bars and the turn of the century, this might be the bike for you.
You know that sound celery makes when it breaks? A series of cracks and creaks followed by a very satisfying "pop"? Get used to that noise because your knees will be making it every time you take a step. The constant movement of your legs and the resistance you have to apply on hills turns your cartilage into jello, and not even the good kind of jello. I'm talking store brand bargain bin here.
This is not what will happen inside your leg
Remember when you would get going way to fast on your big wheel and your feet would fly off the pedals, which would often lead to hard-to-explain bruises on your legs and a crippling fear of sloped surfaces? This is what it is like on a fixed gear bike, only the pedals are made of steel and you are going a billion times faster. What does all that add up to? A permanent limp and mostly useless gonads. Cheers.
In an effort to decrease their life expectancy further, many fixed gear riders take the brakes off their bikes. These bikes are perfectly capable of having brakes, people just remove them in an effort to increase their street cred, in the same way that Hell's Angels don't wear helmets. Nothing says bitchin' like the sweet combination of brain and pavement. Having no breaks means that every time a kid/dog/Spice Girl jumps in front of your bike, the only option left is to use their mangled carcass as a makeshift brake, hopefully giving you enough time to add sarcastic insult to fatal injury.
Not shown: practicality
The original reason anyone in this century rode one of these anachronisms was for their simplicity. Having so few parts means there is much less to break, hence more time at Urban Outfitters. The lack of repairs makes them a favorite among bike messengers, and an insatiable urge to pose makes them a favorite among hipsters.
The only tool you need to fix your fixed. Also works on road rash.
These bikes, when they aren't destroying most of your useful appendages, are actually a really good workout. Having one gear means going up hills sucks, but also makes fitting into your girlfriend's low rise jeans that much easier.
You too can have an ass this nice!
There is also a certain sense of oneness that comes from riding a fixie. Much like Fred Flinstone, you are in constant communication with the road surface. In between making "bone" jokes to the redhead riding shotgun, you actually feel like a part of the vehicle. Man and machine become one, like RoboCop, but with less awesome and much more PBR.