I have been intrigued with the different "facts" surrounding this so have done a little research myself. It seems scientific tests (filter efficiency ,pressure drops etc.) of automotive filters in the public domain are rare. The only ones I could find and reference were three from MIRA (Motor Industry Research Association) in Britain commissioned by K&N Europe in the early 90ís. Also British magazine "FAST CAR" did one in June 96. Here is the interesting bit-they all show "pleated cotton-gauze" filters to be very efficient ! around 97% - this puts them on par with paper- Also they flow well WHEN CLEAN. Foam holds more dust than any other type but also filters poorly and doesnít flow as well as pleated cotton-gauze -read K&N. Paper filters filter well but donít flow well. To summarize: A "pleated cotton-gauze" K&N filter (and I assume other brands of the same design) flow superior to all other types and filter well. The disadvantage is: because the filtering media has a small surface area compared to foam and paper once dirty the pressure drop across them becomes excessive. If left too long as the pressure drop becomes high I think debris will be sucked through, maybe this is where the "they let lots of dirt through" idea comes from. The results seem to indicate as long as they are cleaned often they are superior ! Now I have never bought one myself but in view of these results I am seriously considering it. I have no association with any filter suppliers manufactures etc.