In short, this stuff works, and works incredibly well. Sprayed using a 8 gallon harbor freight air compressor and the Kellsport Woolwax pro undercoating gun. I tend to use 1/2-3/4 of a gallon each spray.
My first two applications I have done were with the original/yellow/clear Fluid film. A major benefit of that is it doesn't alter the appearance of parts very much and you can visibly see what areas may be starting to rust easier. The disadvantage is that it doesn't flow as well and is thicker out of the container, but that potentially aids in keeping it on.
I spray my truck twice a year which is probably overkill (pre summer/beach driving, and pre winter/salt roads). Had I done my truck earlier the first time there wouldn't be a speck of rust to be found. Since my initial treatment, I have no seen any new areas of rust formation. I even power-wash the underside of the truck prior to treatment and still cant get the original coating off. It adheres very well.
I'd say the only major downside is that dust and dirt stick to the coating and can make the underside look a little dirty, and if you go to work on your vehicle it may be a little messier than if it wasn't coated.
Important information:
-A respirator is absolutely necessary when spraying this stuff, goggles too. I wear a full ace 3M respirator when doing this and the overspray gets everywhere. I know it's fairly natural stuff, but it cant be good to get it in your eyes or inhale it. You can see the atomized product in my video, it gets everywhere.
-Clean the underside/frame very well and make sure it is completely dry before application.
-Either cover, or be prepared to clean anything you don't want what is essentially oil on. If this gets on your brakes your stopping power will be reduced.
-Proper application on a Ford F-150 (Inside and out of frame, and any exposed steel) took me about 3/4 hours without prep/cleanup.
-Cleanup: the overspray gets everywhere. Be prepared to cover everything or clean it up. That includes the ground under the vehicle, the outside/paint of the car, and if you do it in a shop... everything in there as well. Highly recommend outdoors on a day with low to no wind.
I find it has been well worth the investment and after 2.5 years of ownership, multiple summers of beach driving, and salt sprayed roads, my frame and axle look brand new.