User Interface I think it looks very good. No numbers on the knobs is good, tweaking by ear is mandatory.
Sound The sound is smooth and round, the filter is even a little sweet and noble. Layers well with real analogs for colors, doesn't have a solid enough kernel to be truly durable with acoustic instruments IMO.
Features Many more sounds than I expected, everything intuitive.
Documentation Considering it's free and simple, I don't expect a printed manual. :) There is a detailed MIDI implementation image at the rgc site, as well as basic information, as much documentation as you need. I hope Pentagon comes with a mini-manual in HTML form though.
Presets Many good presets- and the Pentagon! you have to check it out.
Customer Support Good rep, but no 1st-hand experience here, there's nothing wrong with it so far.
...but Triangle I is a bit of a different animal... I'm glad I have it and I wish rgc:audio wuz still doin' it to the vsti and mini-host world. Does anyone know if they're developing under a new handle??
No. All this series (Triangle, Square, and even Pentagone) is being totally abandoned for years and years.
Like you I have all the versions of all these wonderful synths, and yes they were a bit different in their results from versions to version... But apart very few exceptions I always use the last version of these old excellent synths created by René G. Ceballos (rgcaudio, rgc was the three initials of his name).