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.
Before you criticize someone walk a mile in their shoes. That way when you criticize them you'll be a mile away & you'll have their shoes.
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.
With people like Peter P. around, I am done posting on this website. Mean people have driven me off after 9 plus years. Good luck newbies beware.