Debatable!.
I have one fitted. It makes engine work easier (more space). ButIt does not enble more air get to the engine.
Check past threads on this using the 'search' tool. The stock air box has the advantage of taking cooler ducted air than the K&N which takes it from 360 degree area.
It does no harm and looks cool
But with hindsight, would I have fitted it now after reading all the controversey, NO!.
Ask Kurt B who is now converting his car back to stock so he is not a 'gross polluter'.
rgds Ben