One thing about the stock HD air filter, it is reusable but is rather soft, deteriorates over time and comes apart in bits or can get sucked into the intake passage, especially if cleaned and reused more than 5 or 6 times. The K&N is also reuseable, but the design is very rigid and keeps it's shape and lasts much longer. Price wise they can be pricy, but as Glider said if you shop around they can be found around $35-$45 vs MSRP of $75-$85.