Some swear by it, some swear at it lol
I don't think you're gonna get a clear answer unfortunately as it's an extremely polarized subject. It's proponents deny any and all issues it's detractors would mention so we're left as uninformed as before, with both sides getting a bit silly about it TBH
The best suggestion I figure is just try it for yourself... or not. The problem is ofc trying to ascertain if it actually works when flats are a rare occurrence, like when you just have quality tubes & tires.
My personal theory/practice is pretty simple... I ride as a daily driver/commuter (all seasons) with an absolute minimum of 6000 miles a year on mixed terrain. My cruiser has extremely thick "commercial delivery/rental bike" tires & my winter beater mountain bike has knobbies ofc. I average only one or two flats
at most a year, and those are limited to two scenarios, the first being just a consequence of my slacking & running bald, worn out tires, the second being a catastrophic failure, like a pinch flat/blowout from hitting a pothole at speed, split seam/tear w/e.
While slime might help with the first example, I really don't see it as being worth the bother &
potential problems (out of balance/clogged valve/messy cleanup) as I shoulda/hafta change the tire anyway & there's no way in heck it's help at all w/the second lol
...but know that I'm one of those weirdos that doesn't even like patch kits, I just carry a spare tube. I do have a patch kit for those Murphy Moments, but there's simply nothing that can compete w/a clean, new heavy duty tube in my opinion
