All the "pwr" button does is delay shifting. It doesn't magically increase power. The Camry has both an overdrive button AND a "pwr" button. The OD button is on the shifter, and the "pwr" button is in the center console, under the parking brake lever.
As the informed few in this thread have already said, leave it on. Not doing so is the equivalent of never using the top gear in a standard. =Wastes gas.
Ricky
DesignDawg