My experience with CAI's has been as follows :
1) Your "butt-dyno" thinks they help your truck go faster.
2) They make more noise under the hood, so you're likely to grin more.
3) They drain money from your wallet (cost of the CAI
and through increased fuel usage due to #2).
Mainly, they do 2 & 3, though..
If you want to get rid of the sound suppression from the stock air intake, get a Modular Intake Tube. I've never used a K&N, but I've heard good things about 'em increasing air flow
a little. So, if you want increased air flow for getting
a little more HP when you really wind it up on those WOT runs, get a K&N drop-in..
I think that about covers it.
(Yes, I know most of that was said before. But, I didn't see a reference to the butt-dyno. So, I thought I'd throw that in, too.

)