The Keeley Fuzzhead is one of my favorite fuzz/distortion pedals because of its clarity. I used this pedal a lot when I was in the band "The Everafters." The other guitarist in the band played with a very heavy distortion sound. I added texture by playing my P-90 equipped Flying V using this pedal. It simply sings! The overtones are rich and the tonal clarity was unbelieveable. I really dig the NPN Germanium Transistor and a Darlington Pair of high gain silicon transistors for tons of volume output. This is great for overdriving tube amps or other overdrive pedals. The Silicon/Germanium Toggle switch, this is like a gain Style switch. The "SI" mode inserts two diodes (one silicon and one germanium in parallel) in the distortion circuit so you can have a modern twist on a vintage sound, increases the gain and compression a bit for more sustain. This is more of an overdrive mode.
I strongly recommend this pedal if you love more dirt for your P-90 or single-coil equipped guitar. It's not to shabby for humbuckers either! Check it out for yourself!