My K&N has been fitted for around a year now and I cannot say I've noticed any appreciable difference in performance, perhaps a little more grunt but no more induction roar, which I thought I might get. The main reason I fitted it is because the paper type tend to block up quite quickly in Spain, particularly in the summer when all the mountain tracks are very dusty.
I haven't had to clean mine yet, but when it does become necessary I will be saving money as a paper type would have to have been thrown away.
I paid £32 for my K&N from eBay as against €12 for a paper one from a Spanish motor factor.