I commute exclusively on one of my fixed gears, 4.5 miles each way. There are a couple of small hills along the way, but it helps make it more of a workout. I have a nice geared bike too, but I love riding my fixed gears. After riding a fixed-gear, it feels really weird to coast. I'm going to start riding to take classes that are ~17 miles away, so I may add a freewheel with lower gearing. Oh yeah, I have front brakes only, so I'll probably add some rear brakes when I add the freewheel.