Organ-One could be called an organ simulator. It sounds like an organ, or like a synth organ patch, or...well, it can sound like a lot of things. It doesn't exactly like a real organ, since in the place of clicks, leslie simulations, distortions, and the other virtual organ standbys, it has an envelope generator (for amplitude), ring modulation, and an FX section you can use for flanging, chorusing, and all kinds of other weird effects.
It does have an organ vibe to it, though, definitely. It's rich and deep, rather than bright and shiny, no matter how you manipulate the drawbars. Looking through the drawbars you can find weird, swirly FX and metallic clicks beside the obvious organ patches, but even these weird sounds have a thick, organ-like resonance - even if its more the sound of a synth imitating an organ than a real synth. Feed it through an amp sim and leslie effect, though, and you'll see that it can also make some very believable organ sounds.
Add to all of this a decent GUI thats extremely easy to use, and you've got a VSTi that'll leave you wanting to put organs in your tracks for months!
IMHO... this isn't so much an organ plugin as a "drawbar synth", the difference being that it doesn't do a very good job of simulating a real organ, but it has some features that real organs don't, and can make some useful sounds.
If you're looking for a definitive B3 emulator, this is not it. The stereo FX is basically a chorus, so no leslie emulator here; there's also no distortion or cabinet emulation, and no percussion or keyclick, so you're not going to get some beefy screamin' B going.
You can get some nice pads that are a bit of a change from the usual analog or sampled string patches, and some of those organ-type sounds you used to hear coming from DX7's and D50's. It's got a very clean sound to it like that.
Sound aside it's a stable plugin and the GUI is great and easy to use. There are only a few presets, but then there aren't a ton of really distinct sounds coming out of this thing so I don't expect too many. Many thanks to Bojo for making this one free; it's a good plugin, despite my criticisms about its realism. If you want a realistic B3 plug (for free) check out ZR3.
EDIT: v.2.01 is even better, so I updated my review/rating.
It is a very good sounding organ with many features. I like it very much and don´t have serious critics on it. And it costs far less than the NI B4 organ. For that price you can´t make a fault if you buy it. I don´t understand that it is not known and loved even more.