This article is from the Chrysler-Plymouth-Dodge FAQ, by Dr. David Zatz with numerous contributions by others.
David Cooley reported on a magazine test of aftermarket air filters.
The paper filters were respectable, but the K&N and Accell filters
flowed almost 3 times as much air when dirty as clean paper filters
of the same size. The K&N passed less particulate matter than the
paper filter; as it got dirtier outside, they sprayed on a new coat
of oil (without cleaning) and found it filtered even better.
K&N filters change your engine sound, rarely need replacement,
and flow better when dirty. Other than that, you may not notice much
difference unless you have a high-efficiency exhaust and performance
engine. There has been debate over the actual filtering ability of
these filters; the power boost on TBI cars is negligible.