On an 05? Depends. For performance reasons...not really. The stock one is actually built really well and allows pretty decent air flow. When the K&N filter came out in 04, they actually didn't show any hp gains. BMC filters show like 1hp. Wow. Go for it.
I have a K&N for two reasons only. One, I got it for 20 bucks from a friend that bought the wrong one, and I needed to change my stock one anyhow. Two....I ride enough miles, that I get tired of spending 20-30 bucks for an air filter...when I can just wash the old one out and call it good.