There isn't anything difficult to understand about the Boss FZ-3 when it comes to using it. It's got all of the standard parameters that you'd expect with a distortion, overdrive, or fuzz pedal, and nothing else. Those parameters are for level, tone, and fuzz, and as experienced distortion pedal users will tell you, this is a pretty common make up. Since it's so easy to use, a manual isn't necessary.
While the Boss FZ-3 doesn't have the type of fuzz sound that I usually go for, it still sounds quite good. It's overall a pretty dirty sounding pedal, but you can keep it in check if you want to by keeping the 'fuzz' parameter down at like two or three. I've used the pedal in the recording studio with a Fender Strat and a '65 Fender Deluxe Reverb amp. While the pedal certainly did the job well, there are other fuzz pedals that I like over this one. I look for a whole lot of grit when it comes to fuzz pedals, and there are other ones out there that have more than this one.
While the Boss FZ-3 fuzz pedal will definitely do the job more times than not, I'd encourage those interested in picking up a fuzz pedal to explore your options a bit. There are plenty of fuzz pedals out there, as well as other overdrives and distortions, so having an idea about what you're after is really half the battle. If you do decide you're looking for a fuzz pedal like this, I'd recommend also checking out the Dunlop Fuzz Face and some other fuzz pedals before making your choice...