Beware of the Hype, there are many tests done under laboratory conditions showing that K&N will let thru considerably more particulates than stock paper airfilters, and their perfomance drops off after use and "loading up" Let's look at a quote from one test.
If we had 100 grams of dust on a new BMW filter we would let through a total of 6.6 grams of dust in. If we used the new K&N filter we get 14.8 grams of dust. That's 224% (TWO HUNDRED TWENTY FOUR PERCENT!!) more dust ingested initially, stock vs. "free flow" and this ratio is pretty much held. Somewhere between 200-300% more dirt gets "ingested" anywhere across loading equivalence.
Look at the K&N website, lots of anectodal type "comparisons, Paper/ Foam / K&N" but very short on specifics and independant laboratory data.
Worse they have a drawings showing swirly air on the outside that is miraculously transformed into straight flow as it passes through the filter, CRAP !!! Only a moron would believe flow can be straightened up in the 3/4 mm passing trough the filter membrane. Auto manufactures aren't that stupid, sure they have a cost / Value constraint, but I'm sure if BMW/ Mercedes etc thought a K&N style filter would add a long term performance enhancement and still provide top particulate protection they would adopt the K&N style if not use the product. I am sure K&N gives a slightly better flow rate from new, but in general it has MUCH less filtration area, and filter area is an important part of long term engine protection.