Fumblin46 hit on a very important fact about fix-a-flat ... it will make your tires out of balance, and the next tire rotation/balance will drive the balancer nuts because the tire won't balance no matter how many times he tries (the liquid will keep moving).
I keep fix-a-flat in all of my vehicles for emergencies only, and I always get the tire fixed/replaced as soon as possible after using the stuff. And I make sure to tell the tire guy to expect the stuff in there.
It's good stuff for minor/temporary fixes only.