The K&N air filter has been tested extensively, and if it is maintained as directed, outpaces other type of filters by quite a wide margin in terms of flow. As far as filtering ability, I have never heard that they don't filter well.
We have used them for years on dirt track sprinters, and everything else, and I have never seen evidence that they let dirt in the engine.
David Vizard has done exhaustive tests on all types of air filters and stands by the K&N as the real deal, in terms of filtering and flow.I tend to stick to his recommendations, as I feel his writings and tests are some of the most unbiased research you can read.