I feel it is complete crap and should never be used but.... that being said.... I aways carry it in the toolbox for emergency repairs.
I did use it on a race engine for a season. As previously stated it is a temporary fix and should never be considered perminent. Also agree it is all in how it is applied. I stripped the paint, degreased, used brake clean (no water in the block), heated the surface and spread it wide around the crack I had....
I have a very good welder contact who can weld cast iron but I didnt want to warp the block or heads mid season. (The process uses a LOT of heat to properly weld and penetrate).
After the season was done it was properly fixed with a weld, block and head were decked and pressure tested. It was an SOB to get off...(the JB). I needed to use an angle grinder to remove it.
Your mileage may vary.