I have an Airaid FIPK w/K&N filter on my Ford Truck. There was absolutely no change in performance below 3,000 rpm, and no increase in sound level under light throttle.
However, at wide open throttle there is much more intake noise, but that sounds good to me. Above 3,000 rpm there was a noticeable but not huge increase in power - my guess is 10-15 rear wheel hp. BTW, on the Ford boards, the sales distributor for Superchips reports they have tested various engine add ons and found 10-15 peak hp for the better FIPK systems. FWIW, they also look good under the hood.
It is hard for me to tell if there was an increase in fuel mileage, as my mileage varies quite a bit. IMO, if you have the money and want a modest top end power increase and racier sound, go for it. But if you want more low end power, save your money.
Before adding the FIPK, I had a K&N filter element in the stock airbox. Absolutely no change from stock in power or mileage.