I used it once on a car. It did work on a slow leak (turned out to be a leaky schrader valve that I could have just tightened for the cost of a schrader valve tool -- maybe $2), but it created an imbalance that resulted in a very annoying vibration at highway speeds. VERY ANNOYING, and not immediately apparent what the cause was, as I was just putting around town for a few weeks before I took a road trip and uncovered it.
The shop tried to balance my wheel first, but they could not get it balanced, so they pulled the tire to check. They scolded me for using the slime.
I'll never use it again.