Honestly,I have used it, and just don't think it's worth it. The price and pain in the *ss factor is just too much. It is very picky, requires following the instructions to the letter. You absolutely need to remove all previous coatings and buy the marine clean to go with it. It is actually best to have a bit of surface rust, as it is technically designed to bond with rust.
I would use a good epoxy primer, then lay down a strong top coat. POR15 makes a coating called Chassis Coat Black, that can be applied over primer and other coats. It isn't as tough as the POR15 Rust paint, but combined with epoxy, is more than enough. Chassis saver is another product. Even just a black epoxy by itself will hold up, but may as well put something over it.