I use this Big Muff Pi with a Peavey Millenium AC 5-string bass guitar and an Orange Crunch amp.
This fuzz pedal offers a wide variety of sounds, from the most discret to the most extreme.
Three modes are available: "Normal" is the effect only, "Dry" blends the effect with the dry sound of the guitar, and "Bass Boost" re-introduce some low-frequency content to the fuzz when the tone is set quite high.
There's also three knobs, Volume to set the output volume, Tone to control the overall frequency content of the sound and Sustain to control the amount of distortion.
The "Dry" mode works well with a small amount of Sustain to give an extra bite to your regular bass sound in finger-style. The Normal and Bass Boost modes are more suitable for pick-playing. Normal sounds great with a good bit of sustain and when playing in the higher part of the neck (for Black Dog-style riff for example). But be careful when adding loads of sustain, it becomes quickly very noisy.
I wasn't convinced with the Bass Boost mode, but it's probably a matter of taste.
Hardware-wise, the overall is very sturdy and feels reliable. The major issue with me is that there is no easy access to the 9V battery with no specific compartment. So you have to unscrew the 4 screws at the back and open the box to access it. At that price you would expect that kind of feature. That's why it loses 2 stars in the Features section.
To summarize, a solid pedal with an overall good sound and enough parameters to tailor it to your needs, but the lack of a battery compartment is a big drawback to me, despite the included 9V battery that lasted quite long.