My $0.02 for what it's worth:
On my '97 Escort wagon
(hey! stop that giggling!) I switched from the stock paper filter to the K&N replacement at about 100,000 miles. At about 190,000 miles I placed an aftermarket air intake system with a K&N cone filter
(hey! stop laughing and come back here!). The car suffered irreparable engine damage at 313,000 miles from head failure when valve seat inserts from a rebuilt head trashed all four pistons and cylinders three months ago. From about 100,000 miles I was also using Red Line oil, as I had intended to keep this car on the road for a long while. Unfortunately, heads seem to be a weak point with the Fix-Or-Repair-Daily 2.0 SPI (CVH) engine. Weaselmaster head rebuilders seem to be an even weaker point.
When the rebuilt head was installed earlier in 2006, I saw the condition of the cylinders, and they still had cross-hatching from the factory honing visible after nearly 300,000 miles. Periodically from about 75,000 miles (that is, before I ever installed the first K&N filter), I removed the intake system (stock or aftermarket) at the throttle body to clean the throttle body of oil residue at the plate from the PCV system. This was about every 20,000 miles, or about twice a year based on the usual mileage I would put on the car. When I would check the intake plenum during cleaning, it was
always clean of dust and dirt, despite using a K&N filter after the 100,000 mile mark.
Generally I left the filter alone and cleaned it only about every 30,000 miles or so. The K&N cleaner and water were the only chemicals used for cleaning, and I used the aerosol oil. The cleaner and water was used from the inside out, and the filter gauze would come out nearly white. If I used a little too much oil, I wicked it off the filter with paper towels while blowing into the exit as it was sealed against my mouth and chin. Sounds stupid, but it worked well. The original factory MAF sensor failed at about 180,000 miles, but it was probably time for it to go anyway whether or not a K&N filter was installed. When I had the valve cover off to replace the gasket at well past 200,000 miles, everything in that area was clean, obviously from using Red Line, but my point is that there was no evidence of oil contamination from dirt.
With the K&N filter simply replacing the original-style paper filter, I noticed some difference in performance during full acceleration at highway speeds. With the aftermarket intake, that difference was greater. The Escort was no performance vehicle, mind you, but by seat-of-the-pants feel there was a tangible difference when accelerating on the Interstate. There was little difference with mileage. Would I try another K&N? Sure, why not?
I strongly suspect that problems with K&N filters not filtering or performing properly are mostly from failure to seal the filter against its mating surface, overcleaning, and overoiling. Certain vehicle applications are probably not ideal for use of any aftermarket filter, either, because of plenum routing and restrictions from the factory. Also, manufacturers are doing a better job designing intake systems and aren't really missing too many tricks today. Racing Beat has said on its website that in comparison to, say, 20 or 25 years ago, it was able to get an increase of only a few hp with modified intake and exhaust systems on the Mazda RX-8 compared to the factory setups. Years ago, of course, it was possible to unleash quite a bit of power with such aftermarket changes.
The RX-8 results say something, and they probably apply to many newer vehicles. As a result, unfortunately, it appears that the "maintenance-free" intake system with nonreplaceable filter as on the current US-spec Ford Focus is probably going to be the wave of the future as the automakers attempt to ensure that people won't use
any aftermarket parts under the hood. K&N is a good outfit and I hope it can adapt.