I'm in an 08 x unlimited auto w/ 3.73's on 35's and 3" lift. I've had a superchips programmer for right at a year (bought it at the last Jeep Beach!). During that time I've run it two ways when on road. First, on my 35's I run the 87 octane mode, and in this mode my experience is consistent with about 1.5 mpg savings. Add to that instant throttle response and a noticeable power gain and I'm a happy customer.
Second, when I'm feeling cheep and driving 800 miles a weekend down to south FL and back I'll run it in "economy" mode with little stock tires on stock steelies. That gets me about 16, or roughly 4.5 mpg over normal. It also nics the performance gains and my rig looks stupid with a lift and no fenders on stock wheels. Still, I really hate running down my mud tires on the interstate and I'm cheaper than I am proud.
I'll be honest, I didn't instantly feel a power gain after programming my Jeep. I think I was being hyper sensitive trying to tell a difference. But I did notice the throttle response. I have two "power" mods, the superchips and a k&n cai. I have a buddy with an 08 auto, same lift and tires, but without the programmer and cai. He chirps off the line quicker than me, thanks to his 5.13 gears, but a little bit into second gear my throttle body opens up and I'm gone.
Compared to any other moderately equipped truck or passenger car I'm still slow as crap, but real world testing side by side with a comparable Jeep has proven to me that the programmer really does make a difference.
"Experience is something you don't get until just after you need it." --Stephen Wright