I have been using a K&N on my car. In Canada, it actually was the cheaper option. I paid $75 +tax. The other filters were all $100+ due to shipping and duties.
I used K&N filters on my previous cars and never had a bad experience.
On the M3, it added a nice growl to full throttle induction noises. The car also felt slightly more responsive. My M3 is an '08 that had the restrictive charcoal lined filter.
So I can only recomend it