I use this pedal with electric guitar and bass. I think it would also work great with mono synthesizers. I have years of experience as producer/engineer as well as instrumentalist. Now I work in the MI branch. (No affiliation with this brand)
For me, this octaver is best for bass. Adding a compressor pedal before it boosts its note-tracking performance. It can make synth-like bass with the dry tone set to zero, or set the dry tone to unity gain, then dail in a subtle octave for thundering presence. The stop response time is tight enough to avoid sounding like mud.
How well this pedal works depends on the signal it gets. (True for all octavers like this) With the right kind of guitar/bass, it sounds awesome, for example strat or p-/j-bass. A different type of bass I use which already has a solid, explosive low-end and precise treble doesn't profit much from the sub octave - it doesn't complement the tone. In any case with a compressor before it, you can get better performance by tuning the level characteristics of different instruments so the octaver can work at its sweet spot.
I would improve the design of this pedal with a toggle switch to an alternate sub-tone more suited to some instruments or other sonic textures. How about a clipping waveform? (Less low-end, similar perceived "depth" thanks to low-mid harmonic content)