Initially I wasn't overly impressed with Triangle, but it's grown on me. I liked the smoothness of the sound straight away, but felt it wasn't too versatile. But after using a few times, I've had a good range of sounds from it. It has a very musical filter (in that it will still sound good when self-oscillated) which is not too common in VSTis. I find it good for mid range sounds, and good in the higher range too. What lets it down is the bass response of the filter - it seems to be common in VSTis that it's hard to get filters to work in the bass range. Sure Triangle will give nice rounded warm basses, but it won't resonate usefully down low - it's good for warm bass, but not if you want to do spikey bass with fast decay/attack times.
But I'm being picky really - only classic synths such as 202 or SCI Pro-One have really fast decay portions to the envelope - even Junos etc are a tad slow on that front, and old Korgs are even slower. But Triangle holds up against most common analogue hardware.
In fact, I mainly like it for some of the more delicate sounds - it's not a power house, but careful programming will give some rich filter sounds when you modulate to get movement going in the mids. It doesn't sound anything like a Korg MS series synth, but it reminds me of the type of hollow haunting sounds you can get with those. Which is praise for Triangle - you CAN compare it with analogue synths (unlike alot of VSTis).
The layout is good - obvious and logical with no frills to brighten it up too much.
Not too hard on the CPU, and rock-solid reliable.
The FX are a good addition to the sound - nice chorus and decimator. They complement the patches rather than overpower them.
I don't use it in everything by any means, but it's a good workhorse for those general-duty sounds and some occasional flourishes. Not an in-yer-face character synth, but good quality at what it does.
Quality is the main theme - won't set you on fire, but you can rely on it.
...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).