Today, I used it.
Actually, it was much less dramatic than it sounds. I was making a three-point turn to get out of a dead end in the parking garage and there was a loud pop, followed by a burning smell, and then the smoke started pouring out of all the vents. I got to the nearest parking spot outside (I was leaving the garage because there weren't any), which wasn't easy because I completely lost power assist to the steering on the way. After getting parked and turning the engine off, I grabbed the extinguisher out of the back and popped the hood.
It was mostly smoke. The flames amounted to about three candles' worth, but nevertheless my car was on fire, and I put it out with the extinguisher. It turned out that the power steering had blown out onto the exhaust manifold, which was hot from driving twenty miles. The shop says they didn't get something tightened properly, so they're covering it. Overall, the experience has been about as painless as having your car catch fire can be.
Because I had a fire extinguisher.