Obviously, it depends what you are comparing it to, but David Vizard (extremely credible guy for those who don't know of him) has dyno and bench tested the K&N and found it measurably superior to stock filters.

Rich

 

in which book rskrause I'm almost done reading "How to Build Horsepower" and haven't come across that test yet.
guys check out bobistheoilguy.com good website but like any message board don't believe most of what you read.

http://www.bobistheoilguy.com/airfilter/airtest1.htm
edit:
your right rkrause on pg. 72 he talks about the K&N Stub Stack on two-barrel holley showing no less than 13 hp gains on a street-stock 350. The further I read the more I see your right he does recomend K&N as his first choice showing typical rwhp improvements from 1-5%. In order to see these gains the cars had to be retuned to optimize performance though.
-b

 

the moon could have been in a synchronous orbit with jupiter at that precise moment therefore causing a stronger gravitational pull on the demo machine's ball!

 

I toasted a MAF with a K&N.

Apparently from over-oiling the filter.

I'm telling you, that filter was a light, light pink ... it doesn't take much ...

 

very good pt. mitchntx I believe there was a service annocment on that from the dealership a couple of years ago.