I have one in my VW Passat GLS. Supposedly helps get more air into the the (small 4 cyl, large turbo) engine. For VW/Audis, K&N's are highly suggested if engine mods (chipping) are to be done. With my engine stock, the benefits have been very minimal. It may have smoothed out some of the turbo lag and powerband. They usually cost about $40-$50 & last a long time. You clean and reuse every year or so.
Don't have any plans on purchasing one for the Pilot, unless others have great success with them. I have noticed the slight "flat spot" in the powerband that has been mentioned in some of the car magazine tests. Dont' know if more airflow would make a difference, and not really worried about it at this point.
I have used them in every car/truck/motorcycle I have owned and will be getting one for my Pilot. Performance was improved on every vehicle and I definitely think they are worth the money. They are cleanable/reusable. In the long run, you'll save a little money, fill landfills less with paper elements, and get slightly better performance. When you first install it, don't expect any improvement for awhile as they seem to come overoiled. But after a few weeks, you'll notice a difference especially when you have to gas it hard. The air fliter is #33-2200 per K&N website (please doublecheck before ordering). Lots of information/discussion on it on Acura-MDX.org board.