The Electro Harmonix Nano Bassballs is a pedal with two dynamic envelope filters. Externally it features a 'response' knob and a 'dist' flip switch. Inside there are two trimpots that will change the dynamics and response of the two filters.
I use the Bassballs for guitar, and have rated it for the use with a guitar.
When I first got the pedal, I was very impressed with the light weight but still extremely sturdy construction. I would not be afraid of breaking it even during the harshest of gigs. I started out by opening the back to see if the battery was installed, but when I decided to screw it back up, the first screws threads were broken to the point where it was impossible to get it in. This seems to be a manufacturing fault, but it probably was just a single bad screw, since the other 3 went in there fine.
The bassballs has a surprisingly versatile sound, which can be used with both high gain metal, clean funkyness, or even smooth sections of classic rock. Especially with the internal trimpots.
The distortion switch turns the response knob into a combined fuzz and response knob. This is quite easy to use, but a separate knob for the fuzz would have been preferable. The same for the trimpots, where external knobs would have been preferable, even if they're rarely manipulated when using one instrument.
The Nano Bassballs is a fairly cheap but high quality pedal with a great but specific sound. It's small and should feel right at home on many pedal boards.