I have the Superchips FlashPaq. My purpose in buying the "box" (ECM programer) was to add just enough horsepower to improve the throttle repsonse and make the truck fun to drive. The SuperChips Flashpaq did just that, without any noticeable side effects. I just traded my 1998 12 valve (250,000+ miles) for my 2006 Mega Cab and I am quite happy with how easy (and cheap) the SuperChips product improved my truck.
The box is set on tow, the lowest of the 2 settings (advertised 65 HP); installation was simple and quick. The shift points (auto) are firmer with a definite "seat of the pants" HP gain. My MPG's have improved but I haven't bothered to run the numbers; I am in the oil business and ya'll are paying for my fuel, so who cares? (just kidding).
My truck is stock with the exception of the SuperChips and the EGT gauge. I do not tow anything with significant weight; pontoon boat, jet ski, motorcycle, utility trailer, etc. , so my experience has been slightly elevated (WOT) but in real life terms, normal EGT's.