They 'might' give you a little extra mileage. Not much though...if you even notice.
Power...very little over a clean paper filter.
That said...I have K&N air filters in both my cars. I like being able to clean it instead of paying for a new one. Of course...you have to buy the stuff to clean them.
I know some guys that just run paper filters and replace them when they are dirty. Probably cheaper if they stay clean for a year or so.