I have been using them on my cars for years now, you wont really feel a difference unless you have other mods to support the K&N, but its a nice start.
Lots of people will say they let too much dirt pass through, and really unless you are running baja races or constently in the dirt, and do regular oil changes its a non issue. They have a million mile warrenty so if it gets to dirty, just wash it off and reoil it. 50 bucks to have an airfilter that lasts more than the life of the car.