Well I have a K&N and the hp gain is very slight. If there is any. I can't notice, but it'* a better filter then the paper one, and it allows for a better air flow.
I'd say K&N is one of the best, if not the best. While a stock replacement doesn't add too much hp, a completely different intake is better, with a K&N cone, but of course that costs more $$, unless you can make it yourself. I actually paid like $72 for my K&N at Pepboys
and for the performance gain it'* not worth that. You could probably be creative and make your own intake for less than $100, I think.