They all pretty much work the same from what I read. It all comes down to cosmetics and budget, as raginraj said. You'll hear the whole "heat transmission" argument re: chrome tube (i.e Mac intake), but no one has been able to prove it definitively yet, at least on this site. In fact, many have measured the air temps using assorted methods and they're all pretty much the same with whatever intake you decide upon.
Go with what works for you. I like MAC. Looks cool, was cheaper than K&N and was pretty easy to install. Lots of folks love KKM so I don't think you'll go wrong with whatever you choose. K&N is good stuff, but there seems to be a price premium just because it's K&N.