If by slime you really mean the product, Slime, as in the green stuff you pump in and leave, it's only mediocre in my experience. I've used it in wheelbarrows and lawn tractors and the like. It helped a bit on those that already had leaks. Generally though, after a trying Slime we'd replace the offending tire & tube. Never found it to work worth a darn on bead leaks or tubes. No experience with the pre-slimed or coated tubes.
If you mean generic fix-a-flat cans of goo, I've had them work ok on tubeless tires with a nail hole or such. Very messy, but reasonably effective. Will not work with a ruptured tube.
As for other Slime like prevatives, I've had some work quite well, but most did not.
I still have my tire plugs and tube patches, and they get used. Haven't been able to beat them with any cans of goo. I do tend to like the cans of goo in my lawnmower and wheelbarrow tires and such. Things that get ignored for months until they are flat. Seem to have less problems with them when gooed.