Note that the review link was for a diesel truck site. Diesels run at lower RPM than gasoline engines, and airflow is less crucial at lower RPM. My Boxster @ 7500RPM is sucking a lot more air than my X5 @ 5000RPM.

I don't see any reason to run a K&N filter in a diesel X5. If my X5 was gasoline, I'd check the CFM rating of the OEM filter vs. the aftermarket filter, then find out what the CFM requirements of the engine are at max RPM, and THEN decide if the K&N was worth the cash. In almost all cases, it's probably not worth it.

I run a K&N in my Corvette, but I also have a hacked airbox, free-flowing exhaust, and long-tube headers, so the motor can actually make use of the additional airflow. Switching to a different air filter without making any changes to the rest of the intake/exhaust system is, once again IMO, a waste.