How well do K&N air filters work?

The question posed in the title of this post is one that I've seen debated on the 'net since discovering car-related email lists. Fortunately, a Duramax owner tired of the lack of scientific evidence pursued a path of testing that eventually ended with a lab performing the ISO 5011 procedure on a number of replacement air filters. The result was one that many might have concluded on their own - for a given size, a filter's ability to trap dust is inversely proportional to its flow rate. If keeping dust from your engine is the goal, a paper filter such as an AC Delco will provide far superior filtering performance, but you'll pay a price in airflow. If WOT performance is most important, the K&N filter used in this test provided about 50% less restriction to airflow, with the 1.6" of H20 difference in air pressure drop at 350 CFM making for a minimal difference (~0.5%) in power at this flow rate. The K&N and UNI filters were far less efficient at trapping dust, and note that they lose their flow advantage after filtering about 180 grams of dust.  There's also the economics of washable cotton filters, with a K&N providing a savings of about $285 over 250,000 miles. All of my vehicles currently run K&N filters, but this test has me re-thinking the use of them on daily drivers. Pity that the test didn't include Donaldson filters. [Hat Tip: Impala SS Forum]