I've owned a few of them and they let a lot of junk into the engine (both cars and bikes). Sure they flow a little more air, but at the cost of letting a lot of debris in. This isn't opinion either, there are lots of of people here who have had the same results. Most guys here run the stock filter since it flows a good amount of air and filters very well. Sprint is one of the few that's recommended, it passes some particles, but not nearly as bad as K&N.

Don't believe the K&N hype. HiFlo Filtro makes better oil filters and OEM and Sprint make better air filters.

I'll try to find this website where they did testing on a lot of car air filters including K&N. It was pretty surprising.

Found the data: http://www.billswebspace.com/AirFilterTest.htm