In the TDI community K&N filters are a big no no. Even when properly oiled, they have been known to kill MAFs due to oil contamination. It is also an established fact that K&N filters let more dirt through than most air filters (
http://www.duramax-diesel.com/spicer/index.htm ).
On the other hand it is true that they can improve mileage in a NA car, but at the expense of engine life.