The auxiliary tank is one of my best modifications to date. I'm most disappointed in the FabFours rear bumper.
But this isn't about me crying in my soup over the bumper. IF you are taking the FJ out in the field and running it, an additional 19 gallons of gasoline in a convenient, safe place that lowers your COG makes sense. There is no tank on the back of the spare to splatter with a light rear-end impact and no molotov c0cktails on the roof to fly off if you hit something or are hit (ask MATTMAINSTER about flying cans).
Is it worth the money? Depends on how and more importantly WHERE you drive. There are a number of long desert runs where extra fuel is needed.
To spite my chuckling about Molotov C0cktails, I may keep a 5 gallon can on the roof for give-away gasoline since I have frequently encountered people on the trail who need fuel. I don't want to siphon from the bifurcated filler tube. I don't mind keeping some fuel on hand to help a needy soul.