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Well, here's my real life experience. When I got the engine in my '67 Corvette rebuilt and installed, I got it to the state where it would run ALMOST perfectly. It had an occasional "stumble" when going down the road; not very often, but enough so it was annoying. In trying to track down the problem, my first suspect was a mismatched power valve. To check it out, I installed a temporary vacuum gauge, so I could see how the readings matched up with the actual power valve rating. To get the vacuum gauge hooked up, I had to run the car without an air cleaner. As soon as I did this, the stumble disappeared. To make a long story short, with a stock air cleaner I had an occasional stumble, while without one, the car ran perfect.
The first thing I did was buy a new top of the line filter (I think it was a WIX). That seemed to help a little, but did not eliminate the problem. My next attempt was to drop a bunch of money into a K&N filter. As soon as I installed it, the stumble became more prevalent, enough to cause driveability problems. Took the K&N off, and did a test drive with no filter, and it ran perfect; put either filter back on, and the "stumble" came back, mild with a stock filter, worse with the K&N.
All of this took a lot more time and effort that I am stating here, but I finally came to the conclusion that there was something restricting the airflow. The ultimate solution was to get a premium 14" air filter that was 4" high rather than the 3" that came stock (luckily, it fit under the hood). Stumble gone, 100%. I drove the car the last half of the summer with NO stumble.
I had been chronicling this on the H.A.M.B., and of course, I got all kinds of varying input. The one fact that came through load and clear was that nobody liked K&N filters; not one positive comment. Needless to say, I share the opinions of the first two responders to this thread.