I'm a convert to the K&N units, for 3 reasons.
1 K&N have been making quality filters both oil and air for many years and are a couple of bucks more than other aftermarket units, but still cheaper than OEM, and having checked out a few race teams bikes, and suprise, suprise, fitted with K&N.
2 No other filter that I have seen is fitted with a 17mm nut on the very front, makes for the easiest removal, and fitting, ever seen.
(warning seems way to many peoples she they have a nut on the front, and tend to over tighten them)
3 For the rider who wants more than just a good filter and looks are also, high on the list, (Like me) They also come in a chrome unit, for a few bucks more. ( and still cheaper than OEM)