I started using Fluid Film about two years ago, and I'm pretty impressed. I do find that it does eventually wear/wash off, but it seems to do a stellar job.
Every time I change the oil, I spend my time looking over the entire underbelly of the truck, waiting for the oil to completely drain. At this point, I apply Fluid Film to every stop of rust or bare metal I can find. As a result of this, the underside is quite clear.
One point I would stress is that you really want to shake that can up well. If the weather is cool, I will also stick the whole can in warm water for 15 minutes. If you don't mix it well or if the Dan is cold, you will find the product too thick - it won't flow well.
I'm at the point where I always use Fluid Film during each oil change for my daughter, son and wife. I use about 1/2 a can per car/truck.
Michael