Overdrive just means your transmission will shift up through its gears quicker, and to a larger gear than if it was off. You can leave it on in all driving conditions as long as you are not annoyed by having to force the transmission to downshift when you want to pass, by flooring the gas pedal. Typically, I leave it on unless I'm driving in slippery conditions. Preventing the shift to the highest gear keeps the car from picking up too much speed downhill, where braking can cause a skid. Enjoy!
Your gas mileage will suffer if you turn off the overdrive. Under normal conditions you just leave it on. My Sidekick has the OD kill switch also, and it helps when climbing steep inclines and when I need to get a little more instant RPMs out of the sluggish dog. With 40 extra ponies, I hope that the automatic E has a little more Oomph than the old Kicker or I might have to go back to shifting gears!