I put the spoiler on my Sport Premium. There's no gains mpg/downforce or what so ever. The spoiler is for the purpose of appearance. Some like it, some don't it's up to you. IMO the rear of the car looks a lot better with the spoiler, without it just appears that something is missing.
The spoiler is stable if installed correctly, you can go through a car wash with no problem (other than the antenna).
You will need to drill 4 holes and if you want to remove the spoiler you'll need to get those holes filled and repainted.
So before buying and installing make sure you'll be happy with it for as long as you have your car or until you want to pay a body shop fill and paint the holes.