I kinda disagree with the above. The K&N flows more air over some OEM filters. That does mean that it filters less, so K&N uses an oil to coat what doesn't get filtered. MAke sure you don't over oil, as it may affect MAF sensors.
They can add HP over stock, especially when used with a good headers / exhaust system (IE - banks).
Can you feel a K&N in the seat of your pants? Probably not.
Of more consequence is arranging the intake so that the motor is getting cold air, if you've got an OEM setup that is subject to engine heating.