Airaid, S&B and Volant seem to be the most widely used ones. Of all of those, I have been told the Airaid offers the most performance. It's a monstrous intake. Overall I've seen maybe a .5 mpg increase over the 5,000 or so miles I've had it installed, a minor however slightly noticeable impact on the butt dyno, but above all that is the sound. This thing freaking howls at WOT. I'm still in love with the sound over a year later.
As with any CAI, you're going to see the biggest gains in conjunction with other exhaust mods, so keep that in mind.