Mostly for sound...I personally haven't seen much in the way of MPG improvement. Not sure if anyone has dynoed the kit...but I don't think you're gonna gain much in the hp/torque category either...factory exhaust systems have come a long ways in the past 10 years or so, you just don't see the gains that we used to. My butt-dyno says it's a tad quicker off the line and top end, but we all know how louder mufflers affect our brains... :thumbsup:
It's mainly a sound and cosmetic choice to go aftermarket...if you're looking for big power gains, your money is better spent elsewhere.