First off, I use the Mastotron with an analogue synth so my experience might not be fully relevant for any guitarists reading this.
After settling on some essential time-based effects, I decided to invest in some dirt. I toyed around with a ProCo RAT for a while but it was too noisy and the controls weren't exactly to my liking. So I started shopping around again and eventually went with the Mastotron, the rationale being that it had an impedance control (manufacturers take note), was reasonably priced, and came from Z.Vex, who have a reputation for unique designs with a twist.
I'd seen the photos of course but was still surprised by how small the enclosure actually is; imagine Digitech's new line of pedals, only this one's laid out horizontally, is shorter, lighter, and takes batteries - the Mastotron even ships with one. It's probably going to last you a while too because the draw is a mere 1-ish mA. The knobs have a pleasant resistance to them, while on the bottom you'll find rubbery feet on each corner.
Not only does the pedal look sexy, it also delivers soundwise. This thing can make my synth sing or growl (or crackle if I dial in some white noise)! I almost can't get a bad sound of it. There are plenty of knobs to tweak, which some might find intimidating yet is second nature to me as a synth player. From left to right you've got Volume (output volume), Tone (treble), Pulse Width (hard to describe, you'll have to experiment), Fuzz (amount of dirt), Relax-Push (impedance control although it can do more). My current settings are 10, 3, 10, 4, 10 o'clock. Then there's the invaluable Subs toggle which lets you alternate between three different stages of low end presence. I tend to use 3 the most because it retains my dry tone better but in a band setting with a bassist I can see myself switching to 1 more often.
The Subs switch is a bit difficult to reach spontaneously, which is about the only complaint I have. That and maybe a dry-wet blend knob in the future but that's neither here nor there I guess.
In short, this thing works wonders on an analogue synth. It's accessible price-wise, has vast tone-shaping capabilities, and is small enough to fit wherever.