I've always put K&N filters on all my bikes, mainly because the OEM filters are so stupidly expensive. They do a good job, IMO, and I've had no issues with them, but I don't think they do much other than save me money over the OEM.
I would never use a K&N filter on a turbo-diesel engine due to the enormous amounts of air they consume. Unless that filter is cleaned religiously, and more frequently than normal, it isn't going to work as well as it should. I took the K&N filter, that the owner said had been maintained as required, off my Cummins and could dig chunks of dirt out of the corners and inner pleats. There was no way that the cleaner could have been reused. I pitched it and went back to paper filters, and there's no real reason to use one on a turbo-diesel, IMO anyway.
I wouldn't hesitate to use one on a naturally aspirated engine however.