I have one, but find myself driving with it closed most of the time. With valve closed it sounds stock. Open and its pretty loud. But you can open it just a bit if you like if you just tap the switch instead of holding it down to open - usually what I do.
My section 1 and muffler are stock. I had a RE Section 1 before I put the stock one back on. Not much sound difference during regular driving.
'02 M3 6MT