I've used a fire extinguisher for its intended purpose twice that I can recall right off the top of my head. I worked in a grocery store for about a month. On my last day, my manager, who had already done a number of idiotic things in our brief time together, managed to start a grease fire in the bakery. She asked me how to change the grease in the deep fryer and I told her. She then insisted on doing it a different way, resulting in the fire. I grabbed the extinguisher and put out the fire. Instead of firing my idiotic manager, the store manager fired ME, for defending myself loudly, in front of customers, when the idiotic manager tried to blame me for the fire. She was gone within a week anyway, and I immediately got a much better summer job.
The other time was less dramatic. I was helping a friend soup up a car. He was worried that some modification he had made might cause an engine fire, and he asked me to stand by with the extinguisher in case the engine caught fire while he sat in the driver's seat, revving the engine. He was right: flames shot out of the carburetor and I immediately hit the engine with flame retardant.