Maybe this is covered elsewhere ad naseum, but I find the lack of freewheel helps keep my heartrate up on my fixie. Since you have to pedal all the time, even when you aren't putting the wattage into the cranks you have to keep your legs moving. To me, this feels like more exercise than on my freewheel bike.

And then there are the tarck stands at stop signs... skid and skip stops... and the barspinnzz and wheeliez... lolzerz

Steve