I`ve had mine now for 7 years on my X and have no issues! it will be louder under half to full throttle. not sure on mileage gains as that is not why I have it. I like it for the ease of cleaning the filter and for the increased flow it allows. I even have a drop in replacement in my minivan and have run them in the past with never any issues!
The people that have had issues usually are also the ones that don`t maintain their stuff! they just think they can put them in and forget about everything! the ones with the destroyed engines also probably have the oil changed less often.