I've used it to glue together my Lightning 90MM MAF to the metal adapter flange after it always blew gaskets between the area where the flange bolts to the one side of the MAF.
The problem is that the contact surface on the LMAF is very thin and uneven in certain areas, and the gasket always blows out and leaks boost. I got rid of the gasket and covered the entire mating surface with JB weld. It holds so good, I don't even need to have the MAF and the adapter flange bolted together anymore. The car runs a lot better too.
I know it holds at least 15LBS of boost. I just pulled it out a few weeks ago to clean the MAF, and after 2 years, there is no sign of leaking or deterioration.
For plugging and sealing things it is great. But I haven't had good luck with it putting weight on it. And you need to be very careful how you mix it or it doesn't do well. It can't substitute a weld, like some think.
Good stuff, it definately has its place and good uses.
I used it on my old car to hold the threaded meth injector bung to the charge pipe. Didn't have a welder handy. Worked fine and held at least 10-12 psi.