The Korg Mono/Poly is the most powerful virtual-analog soft synth that I have. The original hardware Mono/Poly was an excellent monophonic synth that was introduced at a time when poly synths were all the rage. To get some of the polysynth business, Korg enabled a polyphonic mode on the Mono/Poly where up to 4 voices could be played with one oscillator per voice and a shared filter section. This type of poly capability was half-assed and soudned like it. It was hard to fault the Mono/Poly as a monophonic synth, however. The software version cures the weaknesses of the hardware version. They polyphonic capability of the software Mono/Poly allows full polyphonic playing of what were previous 4-oscillator mono patches and there are integrated effects. Naturally, with the modern softsynth you have virtually unlimited program storage.
What kind of sounds can the Mono/Poly make? What CAN'T it make? The Mono/Poly can do imitative synthesis, including a surprisingly nice harspichord and an analog brass sound that almost sounds like real sampled horns. It's real forte is gated rhythm pattererns that would be at home in a Deadmaus track.
I usually play the various Korg Legacy synths from my laptop with the little Korg Microkey controller keyboard. I hooked the computer up to my big keyboard with polyphonic aftertouch, and it was a lot of fun controlling the Mono/Poly with the aftertouch.
Every synth has negatives, and the Mono/Poly's negatives are pretty minor. The blue panel is pretty but lack of contrast and small fonts can make it hard to read controls, and the signal flow isn't always obvious. I still get messed up in the loading and saving of sounds. I downloaded the pdf manual, but I can't say that I've spent much time looking at it. I suspect I'm not alone in this habit. It's easy to come up with a synth sound that is "too big for the mix", that is one that overwhelms the song.
Note: the Korg legacy synths including the Mono/Poly are multi-platform. I have run all of them with no problems on a Mac (Os-x 10.5 through 10.7) and Windows 7 pc. Two of the legacy synths, the MS-20 and Polysix have already made it to iOS (iPhone & iPad), so it might mean that a Mono/Poly for iOS might be right around the corner. I have the iPolysix, and I have barely touched it. I'm going to skip the MS-20 and wait for the Mono/Poly.
Overall, the Mono/Poly is a must have for most synthesizer players. Right now the various Korg legacy synths are available separately for $49 each. The Mono/Poly is well worth it.