This is the best free B3 clone by far IMO. The author has put a lot of time into it and it shows. I still prefer NI B4 and prefer a real Hammond over that, but you can't beat the value. It sounds best on single note lines. As soon as you play multiple notes the realism drops a lot, due to the basic design - it's made using SynthEdit.
A real Hammond blends up to 9 tones from the tone generator on each note. But, it only has 91 total tones generated, and the tones get reused all over the keyboard. These tones are close to the natural harmonic partials and the basic concept is additive synthesis. NI was able to get fairly close to a real B3 because their program was written from the ground up, and uses a similar method.
Organized Trio was built using SynthEdit, so the author was more limited in what he could do. This is why it's not so good when you play chords and smears. The tones are not reused across the keyboard by multiple notes, so there is too much of some tones compared to a real B3 when you play chords.
The Leslie effect in the latest version is pretty good for a simulator, on par with other simulators.
OT3 is more flexible in controller assignments than the NI B4, although NI B4II fixes that. You don't need MIDIOX to use Organized Trio as you probably will with NI B4. My keyboard puts expression on MIDI CC 7 or 4 while NI B4 requires it on CC 11. No problems with OT3 there. The interface is great. All the features are where I want them, and all the tweaks I want are available. No annoying animated keyboard. I can look at my fingers for that and I don't need eye candy, I need ear candy.
If you want to get Hammond-like tones for free, this is the one to get. None of the other free ones sound nearly as close to me, and I own a real B3 and Leslie for comparison so I know what it should sound and play like.
The author also has some very good Electric Pianos and a large collection of soundfonts on his website. Very cool!