K Seal is one of those great evils, not because it's a bad product, but because it's results are completely variable engine to engine.
This means it causes a great many internet arguments because person x used it in vehicle A with great success and person y used it in vehicle B and it killed 16 people while burning down the nearest church.
Is it a good product? It is what it is, a product designed to block holes - which it's apparently really good at doing, so it's a good product.
The real question is whether it's a good product for engine A - a question for which answers only lie in the experience of people having used it before.
I wouldn't really demonify or praise it, if you're going to bin the car anyway, or the car is counted as throwaway, it's probably worth a punt.
But it's not a proper fix, it's a limp the car to it's final appointment to a scrap yard move - this may take 3 days or 3 years, but I'd say this product is part of an end game for an engine you don't really care about ever dismantling again.
It may get you moving, but the nature of the product affects every system the coolant touches, so ultimately - it's a calculated risk.