I bought one for my VN and it did sweet stuff all.. But, combined with a cold air intake and better airbox it's a tiny bit more responsive.. I think its one of those "**** everyones got a K&N I better get one" type things...
They are re-useable, but you don't get the cleaning stuff when you buy the filter, you have to buy it all seperate which is crap...
K&N also have an oil filter, supposed to do all this whiz bang crap for like 5 times the price of a normal ryco filter that does the same ****.