Other commenters already mentioned the limited amount of pieces to break or maintain (a big plus). Other benefits:

1. The amount of waste between your legs and the road is supremely limited. Until you've been on (evan a cheap) fixed gear you really haven't felt how much power gets lost in a bike with a de-railer setup. It's very addicting.

2. Weight. Sort of related to above.. The truth is, I like climbing most hills on my fixed better than on my geared bike because it's so light. Gears are nice, but there's no substitute for just plain doing less work.

You can build light fixed-gear bikes, but most fixed-gear bikes you buy at a store are going to be heavier than a racing bike from the same store. Of course, the price difference is going to be $3000, so perhaps that doesn't matter much. I have a steel single/fixed bike for commuting and a carbon-fiber racing bike, and the racing bike is about 7 pounds lighter than the fixed-gear bike. (I do have some bells and whistles on the commuting bike that add weight, like a chainguard, fenders, and a rack; but those only add about a pound or so.)

I'm going to try to build a sub-15-pound fixed-gear bike in the near future, so we'll see how that goes. The reason you see so many 15-pound derailer bikes is because 14.99 pounds is the lower limit for UCI races, and the people that spend money on light bikes are doing that to ride them in UCI races.

As for wasted power; I picked a gear that matches my fixed gear bike's gear and rode both for a mile at a 95rpm cadence. Same heart rate both times, which means my body is working equally hard to propel both the same distance and speed. Pushing your sail-like body through the air is where your energy goes when riding a bike. Everything else is a rounding error.

(It would be nice if someone with power-measuring pedals and a power-measuring hub could try both scenarios, though. Then you would know exactly how many watts are being lost in the drivetrain.)

Sorry.. yeah, I was comparing relatively comparable budgets (or frankly even fixed budget N vs geared budget N*2).

As far as the wasted power, so again.. I don't have a 3k road bike. Both my bikes cost < 700$ to put together, and at that price the fixed gear is probably gonna be lots more efficient :) I wouldn't be surprised to find out that the power loss might be somewhat psychological either. Might be the feeling of the pedals pushing you in the soft parts of the stroke that in-part make you feel that connected with the pavement... either way, it feels good. Probly as good as a few hundred bucks for a bicycle can feel :)

[Edit: Oh! Forgot another thing I really like about fixed's.. Silence! Admittedly, bit of icing on the cake more than a core reason for riding.. but the complete lack of sound that comes from a fixed can be a pretty beautiful thing when riding early morning or late at night (when there's not a bunch of other noise to drown out freewheel clicking anyways)]

Honestly, cheap bike components aren't inefficient, they just wear out very quickly. Low-end components use plastic where high-end components use metal, and really high-end components use titanium instead of aluminum.

In my opinion, cheap components look and feel ugly regardless of what kind of bike they are attached to. The good news is that you can build a fixed gear bike out of good equipment for what you'd pay for a racing bike groupset.

I don't own one and I didn't get it at first, but after riding one I definitely saw some of the appeal. They are very fun to ride in a "I'm totally connected to this bike" sort of way. If you also take off the hand brake, then there's an added element of fun (and terror) because you're using the same mechanism to go and to stop. There's a sort of minimalist joy in the experience.

Really, if you get a chance, ride one for a bit. They're not super practical on hills, but they work surprisingly well because they're so light.

I agree, they are pretty fun. To go fast, you have to pedal fast. It makes it feel like you are going REALLY fast.

That said, I wouldn't want to ride one as my only bike.

The late Sheldon Brown is my go-to resource for all things cycling: http://www.sheldonbrown.com/fixed.html

(Plus his website is awesome.)

Huge props for the Sheldon Brown shout out, never thought I'd see him pop up on this site. Anytime I've got to do bike maintenance, he's the first place I go.

edit: Now that I think about it though, he's the epitome of hacker culture, applied to the bicycle world.

They're fun... (Here is a picture of one that I built: http://thingist.com/t/item/15467/)

But they're kindof a novelty. Mine is a daily driver, but I wish it wasn't (I'm saving up for a replacement).

1) My knees hurt, a lot, from riding it.

2) "A good cadence" on this bike is really really fast. This is good for me, but I'm a social creature, I'm usually riding with other people, and those other people don't necessarily want to be flying around at "oh no I'm late" speeds.

3) Hills. You can't downshift to go up hills.

4) Mine has a brake, so the brake thing isn't a big issue to me. Granted I wish the brake was chunkier, but that's my own fault since I built the bike.

The people who talk about "less parts" are just plain wrong, and are confusing a fixed gear bike with a single speed bike. Fixed gears are a type of single speed, and it's being a single speed that makes things simple.

The bottom line is that fixed gear bikes are fun but, like I said, they're kindof a novelty. My legs have definitely gotten stronger riding mine (her name is Annabelle), so that's a plus, I guess.

You should probably change the gear ratio. I use 42/16, which doesn't hurt my knees but is still around 18mph at 100rpm. Incidentally, my average cadence while commuting is only about 60rpm, so this is probably too high a gear for my actual needs. 42/19 would save my knees and probably be just as good.

You can build a cheap fixed gear bike to be cheaper than a cheap derailer bike. I haven't priced things out, but I bet an OEM can do it for less than $150. The only parts you need are a frame, fork, seat post, saddle, stem, handlebars, one brake lever, one brake caliper, one short brake cable and housing, bottom bracket, crankset, front wheel (hub, rim, spokes), rear wheel (hub, rim, spokes, cog, lockring), and chain.

As soon as you add a derailer setup, you need an extra chainring, rear brakes, shift cables + housing, the derailer itself, 10 cogs instead of 1, and expensive shift levers. (Even Shimano's low-end 9-speed shift levers are in the $150 range. The good ones are $700!)

The real reason for getting a fixed gear bike is that there are less parts to maintain. They don't have all those gears so they can be much lighter. They also tend to not break as often. The downside is that you don't get the advantage of being able to switch gears, which tends to not matter in cities like Manhattan because there aren't many hills and you never reach high speeds for long stretches anyway.

On top of that there's an aesthetic aspect to the simplicity. I tend to find them really beautiful in a post-modern art kind of way.

That's the practical reason. The real reason, though, is street cred. ;)

They give you an incredible amount of control and balance at slow speeds, which is often useful for city riding. They are also really fun in a way that is hard to explain (at least without sounding really pretentious).

Plus they are required for track racing, which is also fun for completely different reasons.

It's really not that practical, but kinda fun. In the summer, you can't rest while going down hill, thus I get waaaay too sweaty. It's also a bit of a fashion statement, if you're into that sorta thing. Very popular amongst certain sub-cultures * cough * hipsters cough If you're an overly practical person, you have no use for them, just continue on, and don't hate.

On another note, in most, even hilly places, I'll usually only use 3-4 out of my 20 possible gears, so getting along with just one isn't too too bad, it's really the inability to coast that gets ya.

Here's some answers about why some people ride fixed gear bikes: http://bicycles.stackexchange.com/q/1980/76 And related to that, single-speeds: http://bicycles.stackexchange.com/q/1983/76