The main advantage of the K&N is that it is servicable, and will outlast your bike with its million mile warranty...besides the airflow that has been discussed...

 

Another thing I have noticed since I added my K&N and Yosh exhaust - the bike seems to be running at a little cooler temperature. Very minor point I know, but yet another piece of info to go along with that "boost" in HP logic! But before doing both mods, I was typically running between 178 & 182 degrees (4th gear highway riding). Now, under similar riding and weather conditions the bike is running around 172 degrees.