First of all it made the whole exhaust sound deeper and louder. It pops a lot more now on decel too. At first I didn't like how much louder it is but now I'm use to it. I have a Stromung though so it wasn't all that quiet to begin with. (I have an AVI sound clip if anyone wants me to e-mail it to them - The one on my image station account isn't working again)
That being said, the butt dyno says the car seems to reach redline quicker and top end is improved. I personally think it's a worth while mod. I've seen info to the effect it adds 14 - 18 BHP but I take things like that w/ a grain of salt.