I have a Mudhoney, Without the boost switched in it's a mild overdrive, kind've like a tubescreamer without the mid-hump, very smooth sounding. Not as much gain as a Keeley TS.
When the boost is on everything fattens up, lots of low end is added on. The top end is slightly fuzzy, but i wouldn't call it a fuzz pedal, more like a really meaty distortion.