Here's my opinion:
I used it a bunch on my old '84 pickup back in '99-'01. There's a paint shop in Nashua that keeps all their products on the shelf, so I bought the whole system, MarineClean. MetalReady, etc. I applied it exactly as the directions said. I went thru a lot of that stuff on that truck, when I first started using it I thought it was the best stuff on the planet. I usually used Rustoleum for a top coat.
Then a year or so later it started peeling off. Some of it I was able to just peel right off with my fingers. In other places, rust was bubbling up underneath, like blisters. So much for that. I've never used it since, and I never will.
Besides the poor longevity, I thought it was overpriced. And, I hated that it basically has no shelf life once the can is opened. I threw a lot of it away, as it just got hard in the can within weeks. I tried all the tricks: storing it upside down, putting a plastic bag on the rim before putting the lid back on, it really didn't help significantly. It seems to react with moisture in the air, so I got the best shelf life if I opened the can, quickly poured out what I wanted into a different disposable container (bottom half of a plastic soda bottle), then put the lid right back on the can, after totally cleaning the lip of the can. If there is any paint in the lip of the can when you put the lid back on, you will never get that lid back off, it is now epoxyied shut. And, like the ads and warning labels say, if you get the stuff on you, there is no way to get it off. Forget cleaning the brushes, just get disposables. I never tried spraying it.
Honestly, I get better results with good 'ol Rustoleum. I get rid of as much rust as possible (flap wheel in a grinder works great), then a good coat of red "Rusty Metal" primer, let it dry for about an hour, then a couple coats of satin black. This is the treatment I did on my frame when I did all the welding and stuff for the SAS back in '04, and it still looks great today. Plus, the stuff is cheap, available all over the place (K-Mart, WalMart, Homey Depot, Lowes, etc), lots of colors, spray cans, several finishes, etc. Touchup with the spray cans is really easy, like I need to do on my sliders now. The only down side with Rustoleum is that it takes a loooonng time to really dry solid, like several days.
There ya go...