What is Slime?
My experience:
6,000 miles in last 11 months on mainly unpaved roads. (MTB) A lot of stickers (goat heads), thorns etc. around here. I have had only two flats that I had to stop and change. I have had probably 6 other punctures that i was aware of that slime sealed on it's own and allowed me to ride on home.
I have probably had 30 other punctures judging by the looks of tubes i have removed from my bikes. Slime usually seals them without me even knowing it!!
Messy? NO! I can remove the valve core and the tube and get virtually not a drop of it on me. If i don get a drop of it on me, i simply wipe it away with a towel.
Verdict: If you would rather spend time riding your bike rather than sitting on the side of the road, fixing a flat, than run slime or goo. If you would rather have to stop every once in awhile and patch a tube or replace it, then don't.
I ride in some very adverse conditions. Often it is below freezing and pitch black out. These are not conditions for trying to do tire repair!