We use it in the shop every week just about. Scrape the rust scale off, use a wire brush to get the rest, prep it with the rust treatment they have and the marine clean and spray or brush the POR15 on. It basically suffocates the rust I guess you can say and doesn't allow it to spread. Works good if you clean the area and get the scale off. You can't just put it over a totally rusted out frame that is falling apart and scaley. I've seen people with it on their frames for years and no problems. I was curious and didn't trust it for a while when I first heard of it a few years ago but trust it now. What I plan to do to my vehicles when I get a chance is do the floors inside and underneath, the inner fenders, firewall, chassis, rear end, inside of the doors and roof, etc and then use their primer and paint the whole engine compartment the same color as the vehicle. Good to do battery trays and the inner fenders with.
Rust Bullet is fine as well but have only used it once. POR15 in my oppinion is a more known product and costs more but they offer more things for it like their primer to use over the POR15 so you can paint over it with no problems.