I've had them on a few different bikes. Their tone is usually lower than stock baffles and a good boost in power from the back pressure. They aren't quiet by any means. I liked every set I've had over the years.
2012 Sunset Red CCT, D&D's, Cobra CVT, Lloyd's Air Kit, Lloyd's ATS, Lloyd's 1/4 Turn Throttle
I run 2 into 1's. I use the supplied cheese grater baffle (or buy one to fit) and mount the Thunder Monster inside of it. Mount it on the engine side so it's the minimum 6" from the end of the pipe for optimim performance. The cheese grater takes the edge off the sound and quites it JUST a little. The Thunder monster helps with torque. I've used this configuration on several bikes and even with an open style airbox for horsepower, the Thunder Monsters bring the torque back up. First time I noticed it was going up a steep grade with twisties. I used to have to drop to 4th, but with the TM's in place I never had to come out of 5th. A dyno backed up what the seat of my pants told me. It's a winning combination!