I use flat bars on my fuji winter fixed gear conversion. I originally got them because I was putting a disc brake on the front and the lever wouldn't fit onto the nitto drops that came with the bike, so I decided to mix it up a little and make the bike more all-terrain with a set of flat bars. I have since ditched the disc brake and now just run the flat bars with a light, a bell, and a pair of oury grips (also the original fuji blue/chrome fork).
I like the flat bars because they give you nice control in traffic, they are really light, really stiff, and you can chop them down to the perfect length. The set I got has a 5 degree bend in them so it gives me a slightly more natural hand position, .
I find I can get nice power out of them, but I have my stem the lowest it can go and could actually use a little more depth (I have been thinking of trying out a set of riser bars, probably flipped, to see how they are in comparison). I do get a bit of arm soreness if I ride for a long time but since I just use it to commute to class its not really an issue.
For posterity:
II would take another picture of it with the original fork, but I have a sickening feeling that somebody broke my digital camera without telling me...