I'll give you my first hand experience. I had one on my truck for a while...thought it was awesome for a while. Started to notice a drop in mileage...about 2-3mpg DECREASE. So I pulled it off and as soon as I closed the bonnet and took a drive I could feel a big time difference in throttle response and an increase in power (according to my butt dyno).

I have been thinking about sending it to Nitro Dave and seeing if he will turn it into a worthless pile of crap into a 150hp gain.