I had installed a series 40 Flowmaster. Before install I could not here any sound out of the tailpipe while driving down the road. After the install there was a noticable sound increase. It was not the high pitch sound of the forign sportcars of today it sounded like more of a lower pitch rumble. Its not the sound of a v8 by any means but it is closer to a v8 sound than a forign sports car sound.
I did notice a slight power increase. It was enough to be noticable. I haven't had it on long enough to tell if there will be a MPG increase but I do not think there will be. I find my self on the accelerator a lot more just to here it.
It is a little louder than I thought it would be, but I will get used to it. In fact it has already grown on me!!!
just my .02