full disclosure, I already do it. Legal or not, if i feel it's the best option for me, I'm going for it. It would be nice to not have to worry about whether or not one of the vehicles you're passing is a police officer, and instead focus completely on the dynamics of the traffic conditions. That said, my first experience with it was in California, following a traffic cop on a motorcycle who seem impressed that I had ridden all the way from ohio. He was kind enough to show me the ropes and lead me back downtown, filtering almost the whole way. Since that experience, I do it when the conditions call for it IMO.