SOUND is the most important thing for me with an exhaust. Even if it gained 50whp, if it sounded like crap, I wouldn't buy it.
So far I haven't heard a single exhaust for the 335i that I have liked the sound of. Most of them either have a weird tone or they are way too quiet. If I wanted a quiet exhaust I would keep it stock, I like to hear my exhaust when I get on it. Make that sucker sing!
I also like the idea of having an option for a quad exhaust, but the sound is the number one priority in my book.