I say most of the exhausts sound good, EXCEPT for D&D. Dont do a D&D pipe unless you want your bike to sound like a 17,000rpm civic w/ no catalytic converter.

Yoshimura sounds good but it is a little on the quiet side in my opinion. If I ever decide to get off my ass and ride and you come out to one of the meets you can hear what a full system on a CBR600 sounds like.