My fixed/SS saga:
I bought a singlespeed mountain bike in '03, after a couple riding buddies started riding them, and looked like they were having way too much fun. I figured I'd just ride it occasionally for fun, and to mix things up. Now it's all I ride and/or race off-road. Haven't ridden a geared bike off-road in two or three years. I'm currently riding/racing a Gary Fisher Rig 29er. Love it.
After getting into singlespeed mtbs, I decided I wanted the same simplicity and ease of maintenance for my grocery-getter commuter, and built up a beater fixed-gear from an old Nishiki frame. Rode it for a year or two before starting a new job that came with a 15 mi. commute. 30 mi./day was further than I wanted to ride an ill-fitting beater, so when I happened across a closeout IRO Jamie Roy, I jumped on it and built that up using the parts from the beater. I ride that, during the off-season, to work and back, and have done a few longer rides on it (Rosarito-Ensenada, rode out to LA for the Bicycle Film Festival last summer, Palm Springs Century, etc.). I haven't done some of the crazy climbing rides, that some of you guys do, on the fixed gear. Having done enough of those off-road, I know that I want the option to *coast* downhill after a monster climb.
A few months after getting the IRO I got tired of stressing about it (It's not spectacular, but it is not a cheap build -- Phil Wood hubs laced to Open Pros, Reynolds Ouzo Pro fork, etc.) while it was out in front of the grocery store, or locked up outside of a bar or something, so I built a new set of wheels, got some new, inexpensive parts, and resurrected the original beater as a grocery-getter/coffee shop/bar-hopping ride.
Cut to a few months ago, and the first FG/SS forum IRO group buy, that I couldn't pass up, and the beater is now upgraded once again. Making my total SS/Fixed stable consist of two IROs, and two Gary Fisher Rigs (Got a new Rig this year, through my race team, after having already bought one about a year ago. The older Rig might soon show up, along with the old, beater, Nishiki frame, in the buy/sell forum.)