I keep mine on all the time and makes for a much better driving experience if you feel your rig is more sheepish than not. My driving is probably 85% city and I get about 12.5 mpg with the PWR on and around 13.5 mpg with it off. This is from driving the exact same route week after week so I'm convinced the 1 mpg is real. I don't mind paying for the extra gas to get a driving experience that's almost sporty......almost.
If you spend a lot of time on the highway you won't notice; as mentioned, it will hold a lower gear longer and downshift quicker to give better performance. It's also useful when towing or carrying a lot weight.