I only use overdrive once I'm up to about fifty, and I've yet to find a situation where I needed the E-AT "power" setting.
The overdrive makes the trans heat up more during stop and go traffic, or city driving, or towing, or driving 4x4. (Heat and automatics don't like each other.) It also makes the truck sluggish at lower speeds (it feels like there's a little old lady pedaling as hard as she can under the hood, especially on hills). It's a highway gear.
Turning the overdrive off also gives you more engine braking when you let off the gas. Let off in overdrive and you coast; disengage the overdrive and the truck slows pretty quickly. My commute includes a couple decent-sized hills, and in underdrive (dunno what else to call it) I seldom have to touch the brakes.
Also, according to my dad, don't use overdrive in snow. Apparently it kicks the gearing around more, making the truck more likely to slip, or something. I don't usually drive over fifty in snow, so I don't worry about it.
(I'm new here too, and sites like this are why I love the Internet. )