The Mono/Poly is the most exciting of the synths you mentioned. Second the awesome modulation. I never use it as a polyphonic synth, although the limitations in its VCA design make it unique when doing so.
What's really exciting is not just that the synth has four VCOs, but that they can be arranged in pairs when using sync and Xmod (all, of course, with control over shape, octave and fine tuning). In this configuration, VCO1 is the master for VCO2, and 3 is the master for 4. This allows you to make some neat, perceptibly bi-timbral sounds if you should desire, but my preferred use of this arrangement is in conjunction with the arpeggiator. One really neat thing about the arp is that you can of course cycle through notes of enormous four oscillator sounds, but if you switch to the poly mode while the arp in engaged, it will cycle through each VCO independently. This is great for things like combining footings and varying octave ranges to get unpredictable "spacey" sounds, and it is also useful to turn the VCO level down for various steps to get a 'sequence' with rests. BUT if you use the Sync, XMod, or the combination of the two with the notes cycling independently in poly mode, you will get alternating steps of clean sounds and nasty modulated or synced sounds. And by the way, you can adjust the LFO to modulate the frequency of all VCOs or just the slaves.
Here's what it sounds like in english:
over and over again. With four VCOs, the pattern length will be 4 steps, but use the full octave range of the arp (3 usually) and the sequence will repeat less often. I don't know... I should just start making youtube videos for this synth because it is one of my favorite monos.