I use mine sometimes to help me slow down more safely from the highway during rain or snow storms, or for climbing hills, all of which the manual says is what the O/D shutoff is for anyways...lol...
I have it off, as well as the A/C or defroster off if I want to accelerate faster from a dead stop without shifting up gears right away (seems like the truck will launch faster from a dead stop with it shut off...thinking that this might give it a closer gear ratio). I generally always leave it on on the highway though for max fuel economy.