I've been looking at these too as being in FNQ, everything is constantly wet for several months a year and things rust overnight. I'm still in the research phase, but from what I've read, they will slow down the rusting process on already rusted vehicles. One did caution that if your paint is in poor condition rust stains may appear on your paintwork due to the formation of a protective ferric oxide coating on any bare metal.
Penetrol also stops rust in its tracks - I have painted all the seams, insides of panels, tyrewell and rust spots with it and will eventually spray the entire underbody to protect it.
Couple more websites: