I can only offer a single anecdote. I had a small external water leak from a 4 cyl aluminum-on-iron engine. This is pretty much the easiest case, so far as head gasket leaks go.
I used the sealant the looks like big gold-colored tablets that you crumble and put in the radiator. About two weeks after I put it in, the leak stopped. Two weeks later I replaced the coolant to minimize the damage that the sealant would do.
It held for over a year before it started leaking again. When I pulled the head to replace the head gasket, I found a lot of sealant hardened in my cooling water passages in the head. I cleaned it out as best I could. The head gasket replacement was about 4 years ago. The car has never overheated so it did no serious damage.