I have never seen any of it work, I have seen it plug up heaters and radiators though.
Is this based on personal experience?
From what I can tell, it's mostly stupid people who think a stop leak will fix a leaking hose, cracked plastic part (elbow, or radiator part), a pump seal, or something else that this clearly won't fix. Now a tiny leak in a heater core might be an appropriate reason to use a stop leak, in moderation. I would not use stop leak for a radiator either, even though in theory it might work for a small leak on the metal part.