For those of you who like to wheel in places like Death Valley or Nevada, with the fine alkali dust there, my recent experience with the filtration performance of the K&N air filter may be of interest.
After about 8K miles of commuting and driving off road in these conditions, I've found that my new K&N filter did not filter out the finer particles of dust. I'm used to seeing a very clean intake behind my OEM filter, but not after using the K&N. There was definitely a light but significant coating of dust behind the filter. It might be OK for most on-road conditions, but in the future, I'm going back to the OEM filter. Just my 2c . . .