You clean it by tapping it against the trash can then throwing it in, or listing it on Ebay and getting an EAO or AEM dry flow. Yes they do typically let more dirt into an engine than a traditional paper filter. All you have to do is take your K&N outside and look through it in the sunlight...there are only 3 layers of gauze and definite visible voids.
I'm sure someone will step up to defend them, but I have yet to see a K&N that I like. Oh yeah and when they finally do get efficient at blocking dirt (i.e. when they are caked with dirt) you have to wash and clean them. That's not the problem, but when you reapply the filter oil in a non-robotic fashion the oil will end up coating your MAF sensor wires in a few thousand miles. Rough running, codes...and you get to pay top dollar to boot. FYI- comb through the UOA section and you can many times see K&N use over time=high silicon.