Personally, I think FF smells horrible and after using it on my former rust bucket, there is a zero percent chance I'm going to slather any other vehicles I own in that stuff. It works great, but the smell is kind of nauseating to me. Some people like it though- smells are such a weird thing. It's also nasty to work around and attracts dust.
Maybe if it was parked outside all winter and I could easily put it on a lift and pressure wash it all off in the spring, then I would consider giving the underside a FF bath. It doesn't smell as much when it's cold, although a fresh coating under the whole underside of the truck is going to reek bad for a long time, cold or not.
Yeah Fluid Film smells not so great, but theres a huge difference between throwing it on underside of your truck and inside the cab. I can always tell when someone went to town with fluid film when I walk into an aircraft.