You can expect around a 5hp gain, it makes the car more responsive and gives it a nice noise although mines rather loud, saying that i do love doing down my windows and listening to the exhaust some nights.
It will make your car quite a but louder - I dont know which one is the best but mine is an OMP Motorsport one and i noticed more power throughout the whole acceleration bracket
Although it is loud... Some times i feel a bit silly when im passing cars on A roads as they look in at what the car is coming past them with such a deep note! but hey im 21 not 35.
Personally i think it makes the car look a lot better, except for the double pipes - they look crud!