I converted a good garage-sale Trek touring bike ($50) to singlespeed with a BMX freewheel and a spacer ($14) five years or so ago, and I love it. I still ride my geared bikes more, but for a quick lunch ride or getting around town, it's my ride of choice. It's not necessary to spend much money if you have or can get a decent older road or mountain bike--just take off the derailleurs, shifters and all but one chainring, replace the freewheel or cassette with a single, shorten up the chain and ride. Took me about half an hour. I put fenders on it and ride it in bad weather, too. Maintenance is done with a garden hose and bottle of ProLink.