So I've been biking as my primary mode of transportation for close to seven years now. For the past five of these years, I've been riding a single speed bike that a friend helped me put together, mostly from used parts.
If you are a biker who uses a bike mostly for in-town commuting and just getting around, and you live in a place without too many hills, single speeds are great bikes. They're durable, efficient, and usually people don't want to steal them (though this is changing as they are suddenly becoming "cool"). They are workhorses, they're fun to ride, and they're easy on the pocketbook.
I've put over 10,000 miles on my single speed in the time I've been riding it, but this past April convinced me that it was time to have a decent bike with gears. A combination of very high winds and muddy conditions made me start to worry about what riding a single-speed in those conditions might be doing to my knees.
So I put up some cash and bought a 1998 Trek 5000 off of Ebay. I'm still keeping the single-speed for getting around town, but when I take longer rides, it will be nice to have something that can handle the Kansas winds a little better.
Here's a picture.

For those of you who like looking at pictures of bikes, there are more pictures
here.
I also shaved my beard this weekend.