Just picked up my '05 1200S from the dealer last night, so this is only my initial impression. I started the bike at the dealership and was initially surprised (and maybe disappointed) that the exhaust was slightly quieter at idle than the stock unit. The ride to the highway seemed to have about equal noise levels as the stock. On the highway, accelerating, the pipe sounds awesome! Noise levels increase, though not greatly, over the stock pipe at around 4000 rpm. I was expecting a high pitched sound, but instead it's a deeper sound (i.e. more bass, so to speak). I rode for a while in 4th gear on the highway just to enjoy the sound. I'm now pleased that it's quiet at idle because I was concerned about waking the neighbors early in the morning. Fit and finish appear to be outstanding, which I guess is to be expected from Yoshimura. Once the wx warms up here in CT and I get to ride the bike again, I'll post more of my opinions.
Chuck