Installed this exhaust on my '04 FZ6 a few weeks ago. Installation was fairly easy, only part I wasn't a fan of was the included fender eliminator kit, since FZ6 has a dual underseat exhaust. I wasn't a fan of the wiring harness for the indicators and license plate lights, and the way the lights double as the bolts to hold your plate on. I just went and bought some bullet connectors so I wont have to cut the wires when I change me plate. The hardest part of the installation was dealing with running the new wires, but it wasn't too bad. Whole entire process took about 2 1/2 hours.
Post installation: It's loud. I love it. There is no question as to whether or not cars can hear you. Its almost too loud at some points, but I just got the Power Tips to help that. If you have close by neighbors and enjoy riding early in the morning or late at night, like I do, then you'll really want to consider the Power Tips if you get this exhaust. It seemed to be running a bit rich, so I get a good whiff of exhaust, especially when I'm idle, and I do get a good amount of deceleration pop with it, but just installed a Power Commander yesterday, so once I get it fully tuned, I'll come back and update this.
In a nutshell: Easy to install, looks and sounds great. Recommend all the way.