The sounds coming out of this things are amazing. If i had to find a word for this pedal, I would say : cotton wool. Every sound coming out of this box is creamy, musical. I know mostly play blues and my fav sound is SRV (not so much overdrive, bright sound, very responsive to the attack). I use '89 lace sensor strat, and a peavey classic 50/410 all tubes at the other end. In between, some pedals (Jen wahs, overdrives/dist, ibanez DE7 always switched to 'echo', an incredible Maestro stage phaser, an aria stereo chorus ACH1 (at home only, because this pedal give its best in stereo) and a T-Rex tremster.
For overdrives, I used to play with a boss SD1, wich sounds good to me, plus a modified bad monkey to reinforce some sounds and have a greater sustain. And a boutique TS808 clone, which is great also and sounds different. But at the moment I put my feet on the FX100, I knew it would be my main overdrive. It sounds so good !! From blues to metal, everything is inside (for gothic/punk, it has probably not enough gain). Can get the same sounds as the SD1, plus so much.
Put the drive to 0 and you will have no overdrive at all (It is probably the only pedal providing this). Turn it to the max, and it's OK for metal. Bass / trebbles knob are not sounding like any other pedal. Their changes are very subtles, but efficient. Once you have the right amount of overdrive / distorsion, the guitar volume knob can be used to go from almost clean -> overdrived.
I give it a 10 because I'm in love with it !! So sweet, so creamy, just like a tube amp but at lower volume, and probably one of the lower noise I ever heard with high distortion. The only bad point if any : not a true bypass - the manual says "active switching for noiseless operation".