improved throttle response. but that's about it in terms of power gains. another member here gained 5whp. not much for the price of the injen...
pluses include a sweet sound, increased gas mileage (if you don't get too crazy on the gas to get the sweet sound), increased throttle response, and sweet looks. people are always impressed by how sharp the injen looks in the engine bay.
negatives include minimal gains in terms of horsepower and cost of the injen. there is another cai system from k&n that runs about $350 or more, so that's even more ridiculously expensive. oh yeah, there's also that remote risk of hydrolocking your engine. but if you are dumb enough to go through a puddle deep enough to hydrolock, you deserved it. hee hee.
opinion: i don't regret buying it. if i could do it all over again, i would.