I don't have any long term experience with Fluid Film, but after much reading and watching a couple of long term review videos from people that have used Fluid Film I decided to go with it. I bought a gallon from Advanced Auto online using a 25% coupon and paid just $32.50. I also bought the
Kellsport Fluid Film Applicator Kit for just $90 with free shipping. I have to say I am impressed with both the product and the gun. The product is very easy to work with and has excellent coverage. The gun sprays perfectly and comes with an adjustable tip plus two extension wands. One wand sprays a 360 degree pattern and a straight spray.
The product appears to be great at protecting already rusted metal as well as clean metal. My understanding is products like this will embed themselves into the pores of existing rust and seal it preventing it from getting worse. Its my understanding that it's also very effective on electrical connections as well.
We haven't had much snow where I am in New England, but from the few times they have sanded/salted the roads I have noticed quite a bit of debris stuck to the fluid film. I'm not concerned with this as I intend to pressure wash my undercarriage annually before I reapply the Fluid Film.
One tip I would recommend is to remove the front and rear wheel well splash shield as well as any beneath the front bumper cover. This allows access to the hidden areas that moisture and salt, etc will finds it's way.
Check out this pic of the front rail on my 2000 Bonneville. This was "protected" by the splash shields. Both left and right were toast.