I know if a vehicle has a blown head gasket it should be replaced. However, I am wondering if anyone has ever used the products that allegedly seal the leak without clogging the engine or causing other issues such as "Steel Seal".

I don't believe in "mechanic in a can" fixes. How does the compound know the difference between a head gasket leak and fine coolant passages?

It hardens when exposed to air?

OP: YouTube sensation Scotty Kilmer has done a couple of videos on these products.

Someone else on here posted about a mechanic that charged for a head gasket repair but really just poured a bunch of stop leak goop in it. I don't know what product was used.

A friend of mine used K-seal and it worked temporarily. None of them will work permanently. Plus they can gum up areas in the cooling system.

I have never seen any of it work, I have seen it plug up heaters and radiators though.

Depends on exactly what is wrong with the head gasket. I recently replaced a head gasket on a beater, and was ready to try a fix in the bottle until I pulled the valve cover gasket and found a broken head bolt, and two loose head bolts. A sealer isn't going to fix that. However, all I otherwise had was an external coolant leak and yes a fix in a bottle would have otherwise fixed that.

https://priuschat.com/threads/stop-leak-will-come-back-to-bite-you.211890/

But a flush with an acidic product will supposedly dissolve it.

I tried k seal several years ago in an old ford Taurus 3.8, was a beater car for work....temporary fix at best

It may make sense using this product if you want to unload the car to someone else or if you want to squeeze a few more miles out of a junker.... but not for a long term fix.

Kestas's comment applies to any "quick fix" IMO.

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.

I believe Chris wrenches on automobiles for a living. His experience weighs heavily here.

And how many of those people put in too much, or were trying to fix something like cracked plastic on a radiator and kept putting in more, and let air get into their system, put in more than one kind, were trying to fix a large leak, etc.?

Many of the block seal type products have fiber material that can clog radiators. The Blue Devil brand doesn't and is all liquid. I've used it on two cars with success but you have to follow the directions exactly. It's not something you just pour in the radiator and drive off. Many people who dismiss such products have done so only from word of mouth and have never actually used them, or used them incorrectly.