If you have a Honda S2000 that regularly sees 9000 rpms you might notice a performance difference. However, on most cars driven under typical conditions the difference will probably only be in sound of the intake, if any. The main benefit to the K&N filter under typical applications is the longevity of the filter. It likely will pay for itself over the life of the car, but you need to be very careful to not over oil it after cleaning. If you over oil it, the excess oil can easily accumulate on the MAF sensor and cause fuel economy concerns. Most folks agree that the airflow benefit happens only at very high rpms, ranges in excess of typical urban driving.
-Rod