I just started commuting fixed and it forces you to cycle differently.
I find the fixed bike extremely manoeuvrable which I find gives me some confidence in traffic. I assume that doesn't apply to single speed.
As
@smokeysmoo says, you have to read the road better, or it MAKES you read the road better. I am trying to use the brakes only for situations where I am unable to control it naturally. My hand floats over the lever near constantly. After using the fixed, I realise just how reliant I am on my brakes on my regular commuter. It almost begins to seem silly that I would be seriously injured a dozen times a commute if they failed at any of those points. You read everything more. This may just be fear of the unknown for me right now, but it is a buzz. It sounds dumb, but I feel like I am learning to cycle... 30 years after starting.
I expected it to do my knees in more, but it hasn't really - but I am not leathering it yet - still learning the ropes. I would say it is relatively agressively geared, and ideally I would drop a tooth or two at the front or add one or two at the rear, but this is an experiment for now. I do have a couple of climbs, and I can make it. It can take a bit of effort, but I do wonder what I need all those gears for. I still feel the climb shifting though my gears...
What I do like is how solid it feels. I have come off my bike due to chain drops during bursts of acceleration, once the derailleur disappeared into my spokes... just over the years I have had more mechanical inspired injuries than any other category. Maybe it is my bad luck?
Learning to control your speed downhill is important, so you don't look like a toddler on a trike that has started moving too fast. *legs up* *whee*!
If someone was to thrust a Langster at me, I would probably hang my commuter tank up. As it is, the fixie I have is an utter lemon - but it has been entertaining.
A consideration is lot of cheaper fixies will not take mudguards/rack, so bear that in mind for a commuter bike.
When I rode my regular commuter after a couple of days fixed, the lack of feedback when I was freewheeling FREAKED me out. After the first day of riding fixed, my legs wanted to walk backwards because they were FREAKED at having to use these forgotten about muscles.
Was any of that helpful?