I have one (did not cost me a dime, so why not try it?).
I had used one on my 2005 Dakota 4.7L, HUGE difference in conjunction with other mods.
On the 4.0 in the Jeep, not so much. Quite honestly, I did not expect anything shocking as comparing a straight 6 to a complicated V8 is comparing apples to oranges...
I must first say I drive the Jeep like a granny anyway. I did not buy my Jeep to race around in. Everything is stock, minus a dynomax exaust (stock air filter and box).
91 Octane mode: Slight increase in power, mileage stays the same. When I mean slight..I mean I notice it on the bottom end through the entire powerband, but it ain't much.
Not worth the extra $$ per gallon for the minimal "feel".
87 Octane mode: engine idles less smooth. mileage is the same as stock. Does it feel different? Only at idle
Econo mode: Engine idles as stock. Feels like it has the same power as stock. Mileage is slightly better (maybe 1-2MPG, but my driving conditions were not 100% the same).
My conclusion: Save your money. Don't blow it on this, as it will not meet your expectations.
Why I keep it: Hey, it did not cost me anything, and I like to use it to read codes and use it for diags (but there are better options out there for this).
Also, I have a manual. If this could be used to modify shift points and speedo calibration, then MAYBE worth $100.00 to do this along with read codes.
My honest to goodness opinion based on my experience.