I ride a BMX-type single speed around the trails here. Love it. My wife has a $80 walmart special that she's been using for a couple of years, seems to work fine for her. We have about 70 miles of concreted "greenbelt" that we go on, which I ride with no hands because it makes me work harder. I have no idea how many miles we cover, but in the texas heat it doesn't take long to wear down
I'm saving up for a mountain bike at the local shop, about $750, but it sure felt nice when I tested it.