In terms of hardware synths, I have a microKorgXL and a Moog Slim Phatty which I love to pieces and use all the time... However I don't think either of them get as much use as my montron and I think that is purely down to the in built speaker... I have yet to use it in any of my productions and purely play around with it for fun, but I would like to include it in a tune soon. It is sooo much fun for when your chilling and wanna play around with the possibilities of what you can create. I bought this and my housemate loved it so much he went and got the original monotron so we now chain them together and run them through our guitar pedals and kaoss pads to see how far we can push the sounds... if you are a tech head you will know that although sometimes utterly pointless, this experimental process is a lot of fun.
Specifically for the duo: I have found that it is definitely the only real monotron designed for playing melodic lines due to its quantizable ribbon keyboard. If you have a chromatic tuner at hand it is very easy to set a key and a scale and get some sweet lines and hooks out of it. The second oscillator is wicked for creating interval based sounds which are reminiscent of retro video game music and electro due to both oscillators being square waveforms. The x-mod on the duo is essentially a frequency modulation effect that routes the output of each oscillator across one another other (hence cross or x mod). The second oscillator can reach inaudible low frequencies and therefor the effect can also be used as an LFO using the square wave of oscillator 2 to modulate the pitch of oscillator one. When turned up to a higher frequency, oscillator 2 modulates the first oscillator to create harmonically rich tones not present in either oscillator creating some interesting distorted and metallic sounds.
I am definitely going to buy more monotrons once my next load of pocket money comes in to chain them all together :)