The main adavantage of a full K&N/MAC/BOMZ(ebay) type unit is they have a conical filter. Conical filters have more surface area to pull air through; so in theory, they have a bit less restriction, and need cleaning less often.
I put the "full" MAC intake on mine, and it has been fine. The MAC is every bit the quality of the K&N, and about $75 less. BOMZ is another brand that I have seen marketed over ebay, you can land one of them for about $30. The BOMZ adds a conical filter in place of the airbox, but leaves the factory tube. As ions said, the factory tube is probably fine.
I was going to get a BOMZ filter, then I had a good deal pop up on a used MAC (with extra filter) from a MYST.com member, so I got that instead.