The toilet in the bathroom I was using in my mom's house was running. Not a "jiggle-the-handle" kind of running -- more like an "unstoppable-gallon-an-hour" kind of run. I lived with it for a couple of days, thinking, "If I were back in Seattle I'd go to the hardware store, buy replacement innards, and install them." But my mom's tool collection is a bit sparser than mine (no pneumatic tools, no table saw. Imagine! And her an 80-something year old woman!).
Then the light went on: hardware stores also sell tools. Two pairs of pump pliers and a couple of hours later, no running toilet.
Oh, and the review on Amazon of the Fluidmaster 400AK doesn't reflect my experience. I've used them several times on different toilets and they've always worked.