They work good, I use it in combination with a manga flux test to make sure that I am working on a good block. I wound use it after cleaning the block, as crud in the water jackets may seal a crack, giving a false sense of security.
I found a defect in one block that other wise would have been missed. Somebody in the past had broken a bolt off in the back of the block, in the process of drilling it out, they drilled thru into the water jacket.
