The Excursion we have has one and I personally think it is a good motor. We have yet to have any problems and it tows well and doesnt get that bad of gas mileage, however ours only has 70k on it.
My brothers is in a 99 f250. I think all it has is a k/n filter and a tornado. He says the tornado helped his milage but I put one on my explorer and it didn't do nothing that I could tell. I do know though that when we take it out on the interstate and run 80 the milage gets pretty bad.