I've never used it in 45 years of riding. I think it's be messy, and there are too many other remedies for frequent flats, e.g., belted tires, thorn resistant tubes, heavier tires, careful riding, etc.
You know that little thing in your head that keeps you from saying things you shouldn't? ......Yeah, I don't have one of those..
Nothing like seeing slime all over an expensive road bike after a blow out. We were out in the middle of nowhere and the bike couldn't really be cleaned up. Then the guy had to fix the flat with all the green goo everywhere. Not pretty. Not practical.
"I am looked on by others as an eccentric...because I run errands with my bicycle"