I had one on my truck. Loved it. It was quite loud when towing though. Last year we went to Florida for our yearly trip and the loud exhaust kept my 16 month old daughter awake the entire drive. When we got home I took it off and put the stock exhaust back on.
Modern engines are very well designed as they leave the dealer. I doubt you will recognize much gain in power by adding an exhaust and may actually lose some low end power.
The main reason to change exhaust is for sound and looks. That's why I did it. I miss my exhaust when I am not towing... it wasn't worth keeping my daughter awake on longer trips though.