I've used one on my 2008 JK for a little over a year. It does give you a little more get up and go on the low end. Used to believe I was getting better gas mileage, but I'm not sure anymore. Recently went back to stock and the mileage seems better at stock. Might just be that we are getting summer blend fuel now.
It does have other nice features if you have an auto transmission (adjust shift points), but I've got a stick. You can also correct your speedo if you have larger tires, adjust throttle response and a few other bells and whistles.
The trans shift points is the most noticeable feature. I have one and I like it but don't expect power and milage gains. Look at the features. If it has a few you will use( like tpms, gear ratio, tire size, speedo,,adjustment) it's worth it.