I've got one on my MY06 STI

If you want to hear it I've uploaded a vid on Utube with externals and interal sound.

http://www.youtube.com/watch?v=rbxzY_br4eo

Pros are obviously, Cons however, the bastard keeps seizing up, have to take the muffler off at least once a fortnight to free up the valve. Now I've just give up and left it full open, might as well have bought a normal cannon.