I thought I'd share my limited first experience. I just rode to the bank, about a five minute round trip at most, after flipping to the fixed hub on my new single speed bike.

Like everyone else has said, it is weird. It was such a short ride that I didn't get much of feel for it. The first thought I had was "oh ****! I can't stop!" Of course I have brakes, but I had a sense of momentum when I hopped down a curb or had to slow down to a stop and couldn't coast to a stop. Going down hill was the scariest, especially because I have no straps or clips on my pedals. I love going really slow on the fixed hub, especially on the sidewalk when there are a lot of people. There's so much more control.

I'm going to keep it a fixie for now, partly because it was such a pain to switch it over and get the chain tension right.