If you don't add things like this to the body of your car, then you won't attract haters that have a negative opinion. Nobody will bust on your car for having a stock, straight and clean body, trust me. You don't know me to trust me, but that's really the best advice I can give. Your stock bodied car will never go out of style when the styles change or when the next fad rolls in.
Really, a GOOD paint job is way more impressive than aftermarket body parts that are painted to match. The majority of compliments I've recieved on my GSX have come from enthusiasts of ALL kinds that say "it looks really clean" because I left the body bone-stock. If there's a factory part that equals what you're looking for, chances are its form and function are already tested, proven, and the fit is better.
I personally think that rear window spoiler looks terrible. I don't necessarily think it's rice, but you don't have a Camaro. The line from the roof to the rear A-pillar just doesn't flow. Maybe if you paid a few hundred dollars to have the thing reshaped to cover the rear window's seal and molded into the body, then maybe... but I think it's a waste of money that would be better spent on wheel locks or suspension parts.