The Monotron Duo is a cool little box with plenty of fun sounds in it. Just don't expect too much from it. Adding a second oscillator and an x-mod to the original monotron design provides more cool sounds and possibilities. The filter is the same and sounds very juicy and breaks up really nicely at extreme resonance settings. Plus, you can use the filter to mangle any outside source you plug into it too! This is a real analog synth, so it sounds very alive and a little unpredictable.
I feel that it's a bit of a mix between a toy, and effect box and a real instrument. The filter, the oscillators and the sound are great, but a couple of things prevent me from taking it too seriously. Firstly, the little ribbon keyboard is pretty hard to play if you want to play melodies with it. The second thing is the noisy output, leaving a moderate amount of hiss to everything coming out of it.
I'd describe it as a cool gadget that can add spice to your music, or be fun to just play around with and create all sorts of weird and wonderful sounds. Can't beat the price considering it's fully analog!