In terms of the specs, Korg really only tweaked them. As the name implies, it's smaller—by 14%, to be exact—which could make the keyboard more difficult to play (though as a pianist, I didn't have a problem). They've also provided very basic MIDI/USB functionality: notes only, with no velocity. The manual states that the "circuits of the MS-20 mini are nearly all reproductions of the original MS-20." The VCOs and VCFs are exact replicas, whereas the VCA retains the basic architecture while making changes to improve the signal to noise ratio. This is moot, though; what matters is how it sounds. The general consensus seems to be that the Mini not only recreates the sound of the original faithfully, but will also drift less than a vintage unit. (While I don't have an original MS-20 myself, a Create Digital Music post from earlier this year offers A/B samples that help hammer home this point.)
The MS-20, and now the MS-20 Mini, has a very particular character. It's very raw—less warm, more buzzy and biting. (Legowelt once described it as sounding like "battery poultry diarrhea". Since he's used the MS-20 on a number of his tracks, we can assume that this is more a statement of tonal quality than a damnation.) It's particularly good for sound effects and for the kind of mix-cutting leads and basses that are halfway to sound effects. It also yields powerful drum and percussive sounds. It's not so great for rich, low basses, and you can pretty much forget about it soft, evolving pads.
The semi-modular architecture is important in making it lean towards these kinds of sounds. On the left of the panel are the basic knobs—and it'll happily produce a range of sounds with these alone. On the right is a small patch bay that greatly expands its capabilities. The unpatched setup is a straightforward oscillator-to-filter-to-amplifier signal chain. Its two oscillators each offer a basic selection of waves, as well as ring modulation, and they're pitch-adjustable. There are pulse waves, though it can't do pulse width modulation. There are separate high- and low-pass filters in series, each with resonance, enabling band-pass filtering if you set them both to the same frequency. The filters self-oscillate, a fairly uncommon feature that can be used for fantastic pitch-shifting whips and weird sine-y FX. There's an AHDSR envelope hardwired to the amp, as well as to the filters with adjustable amounts. An LFO, which fades between a triangle, ramp-up or ramp-down, can control the cutoff of the filters or the pitch of the oscillators. A second, much simpler envelope can control the pitch of the oscillators, too. There's also portamento.
The patchbay section isn't too complicated—an hour or two with the manual should be enough for anyone with a decent working knowledge of subtractive synthesis. (This feature is also a great way of getting a feel for a synth's signal path, which is why it's a staple in Rob Papen's video course.) The envelopes can be routed in a variety of ways, as you might expect, and you can also invert them. The modulation generator, though, is where the fun really starts. It can become a square wave with variable pulse width; it can be run through an amplifier and given an envelope that makes the modulation decay or grow; or it can be used along with the white/pink noise generator to give retro sample-and-hold burble effects. The noise generator can, obviously, be used as a sound source but also as a direct modulation source. You can use the modulation wheel and trigger button on the left of the keyboard to control the envelopes. The keyboard's pitch and trigger control voltages are also available to route where you see fit.
There's also an "external signal processor" section at the bottom. The chief use of this is to convert an external signal—from an electric guitar, say—into a pitch control voltage, an amplitude envelope and a trigger voltage. These can then be used to play the synth itself, after setting up the patch as usual. These voltages can also be routed anywhere the input is suitable, and it goes without saying that in all of the discussed areas, wild experimentation is possible.
The ascetic, hard-edged black-and-white stylings of the MS-20 have found a certain retro, minimalist appeal. Moreover, its look nicely reflects its sound. The hardware feels as bombproof as it looks, with the controls having a smooth thickness to turn, and with a healthy proliferation of metal in the case. It hammers home the fact that Korg haven't messed up their mission. More so than with most synths, this one is all about the sound. It's not the most versatile, but if you're looking for something that's both iconic and eyebrow raising, this is your beast.