Rich, punchy, warm, and spunky. I would have a hard time saying it was a software synth if I heard it in a blind test.
I notice that the filter envelope takes a minute to begin functioning when you first load a project containing Mono/Fury or when you first load the VST into a project. Caused me subtle alarm the first time I encountered that.
Mono/Fury is my kind of synth - straightforward, easy to use and with a great sound. I bought a brand new Mono/Poly in 1982, but I cant remember exactly what it sounded like, so I have no idea how accurate this is as an emulation and, frankly, I don't care. What matters to me is that it sounds fantastic and it is very easy to get the best from it with minimal effort. It also uses practically no CPU at all, as in the CPU meter reads 0% most the time when I am using it and I've never seen it go over 2%.
For those unfamiliar with the Mono/Poly, it has four VCOs that you can use to play separately, giving you four voices of polyphony (although there is still only one filter) or you can stack them for a fat unison sound (monophonic). Hence Mono/Poly. In it's day it was one of the best synths money could buy, easily able to compete with things like ARP's Odyssey or the MiniMoog and better than the instruments being produced by it's Japanese competitors. It had a section devoted to oscillator cross-modulation which, combined with the characterful filter, gave the Mono/Poly a lot of versatility and a very aggressive sound if you wanted it.
Mono/Fury definitely captures the aggression of the original. Dial in a bit of X-MOD and/or FM and use the Filter Envelope to modulate everything and off you go. Turn up the resonance for even more nastiness. It goes further than a lot of synths are capable of without adding distortion somewhere in the signal path. You might think it would sound a bit weak or hollow in Poly mode but you'd be wrong. The oscillators have a big sound and hold their own in a mix.
The sound is brilliant but it's the usability of Mono/Fury that really makes it great. Unlike Korg's own VSTi version, Mono/Fury has a simple layout. There are more controls if you need them, accessed via the little icon in the lower left corner, but they are hidden by default, leaving you with just the controls for the synth engine itself. It's a stroke of genius that makes Mono/Fury so much easier to work with than Korg's emulation. The resizable GUI itself is also nicer than Korg's, even their revamped versions.
Mono/Fury holds it's own against the best emulations out there. If it was $99 it would sell really well, I reckon, and maybe it would be taken more seriously, too. Make no mistake, it is a great synth that just happens to also be donationware. If you don't give the developer some money for such an amazing plug-in, you're basically stealing. (For the record, I donated 30 Euros for this and I intend to make similar donations for any of his other synths I decide to keep.) Mono/Fury is my new go-to synth. Give it a go and you might find it becomes yours, too.
P.S. If you want a good companion synth to go with Mono/Fury, I'd recommend you check out Nabla, FBM's Korg Delta emulation. It's a polysynth with a very different feature set but still with the great Korg sound from that era. They go together really well.