The Korg Mono/Poly is an interesting synthesizer. It lives up to the name, where it tries to be a monophonic or polyphonic synthesizer, depending on the mode that it is in. Honestly, I never use mine in polyphonic mode unless I am using the arpeggio. I think that it is pretty weak as a polyphonic synthesizer, mainly due to the fact that it only has a single envelope for the final output VCA. This means that when you play a chord with one hand, then a single note with the other hand, it re-triggers the chord as well.
Now onto its great features: As a monophonic synthesizer it is top notch! It has 4 oscillators that can be stacked to make for a very full sound. There are some cross modulation and hard sync options (under the "effects" options) that really add to the sonic capabilities. You can sync 3 oscillators to the first and get incredibly harmonically rich sounds, or you can have them free floating and get incredibly thick, beating sounds. When you sync the 3 oscillators to the first, it is preferable to put the Mono/Poly in polyphonic mode, play the root note first, then the synced harmonic notes, and while you are holding these, press the chord button. This way, you can sync various harmonics to the fundamental much like an additive synthesizer. It also has 2 LFO's where you can have a slow one control the PWM and a faster one control a vibrato, for instance! It is packed with great features!
Though the filter is very nice sounding and goes into self oscillation, I wish that there was an option for high pass filtering. The filter is only a low pass filter. There are ways to mix a non-filtered signal with a low passed version (through an inverter) to make a high pass filtered sound. I have often thought about putting a switch in for each oscillator to add a final mixer where you can add an inverted raw oscillator signal back into the final VCA, and therefore be able to select high or low pass filtering for each of the 4 oscillators. I haven't done this mod, though... and I am not 100% sure that it would work - it would depend on how the phase is out of the filter in comparison to the raw oscillator signal.... anyway...
I have heard arguments against the way it mixes these oscillators together, saying that the summed amount doesn't get louder, but just changes the tone - but I think that the way it sums everything together sounds very smooth. In fact, I think that I prefer the way it sums things because you can add oscillators to thicken the sound and not really make it louder - it is almost like it has a compressor or limiter as part of the mixing circuit.
I mentioned the arpeggio briefly - but one of the strengths of the arpeggio is when you turn on the arpeggio, then put it into polyphonic mode. You can have each note of the arpeggio play a different oscillator/waveshape/octave - and then change them on the fly.