I just put on a K & N filter on my kids 04 crf 50 last weekend, Ran fine and a lot easyer to clean! About the same noise level and my kid says it seems faster!
I've had the K&N, Takegawa and I am now ordering the UNI pod filter.
One thing that really sucks about the K&N style, it seems to get wet/mud on it easier and when it does, BBBOOOOOGGGGG out.... takes a while to dry.....