I've used one of these for a year on my motor. Not a K&N but one of those green cotton jobs, which costs £25. I think the car runs smoother and revs better, but have no evidence that these things improve MPG one iota. Oh, and yes, you won't have to buy new filters, but they are a bit of a pig to clean properly, apparently. However, I checked mine out the other week, and it seemed to be very clean and just needed a tap out.
There are well populated threads elsewhere on the site about pros and cons.
Edited by: romseyraver
2004 1.6 petrol LX