I converted an old Trek tourer about three years ago, and I love it. It's not the bike I ride most (I live in big mountains), but I always have fun on it.
FWIW, the whole deal took a couple of hours on a Saturday and cost less than $20. I already had the bike, a Trek 520 (pretty decent ride when it was new, and my main bike for several years), but didn't ride it much since I bought an Atlantis. I took off everything I wouldn't need for an SS, including the inner and outer chainrings from the triple crankset, then spun off the old freewheel and put on a one-cog BMX freewheel ($13.95) and a spacer I got from a shop that specializes in BMX bikes (a buck each, and I only needed one, but I wasn't sure so I bought four). The rear dropouts are semi-horizontal, and that gave me JUST enough adjustment to get good chain tension, so I didn't have to use a Singleator or some other tensioning device. You can also fiddle tension in a narrow range by changing the cog--one tooth more or less can make it work sometimes. There's a lot more on this at
www.sheldonbrown.com, if you haven't checked that out, and you can buy single speed "kits" (cog and spacers for a cassette hub) from Excel Sports for a few dollars. It's an easy conversion and lots of fun to ride.