Been playing around with it the last few hours, and thought a description might help the people that keep asking.
From what I can tell, ITER is a basic sine/tri sounding tone (kind of Rhodes-y). What makes it interesting is a weird resonant filter/feedback/env circuit. Controls as far as I can tell are:
BLUE: Filter env. amount (like “contour” on a minimoog)
GREEN: Filter env. speed (min= quick, max= slow)
WHITE: Cutoff (only works with BLUE turned up
RED: Filter cutoff modulation amount (sounds noise-based to me)
So, with all dials at zero, you just get a basic sine/tri tone. Messing around with G, W, and R does almost nothing without turning up the BLUE control. Which is weird, and probably why it feels a little crappy when you first use it. But, with a combo of the amp env you can get some multi-stage env. shapes that are otherwise impossible on the OP1 as it only has the one envelope usually.
SO, in a way, ITER is kind of like a Volca Kick - a resonant filter with a filter cutoff env./modulation. It can make some MASSIVE kick sounds, and bass sounds in the low registers. In higher registers the RED control makes it pretty unstable/random/noisy sounding at high levels, but functions really well to give that weird organic vibe to the following notes as a rhodes-y kind of sound.
PROS: Good Rhodes sounds, really thicc bass sounds, weird organic vibe due to the noise-based modulation, psuedo-dual-envelope when combined with the amp envelope
CONS: bit weird that nothing does anything without the BLUE knob up, envelope is ONLY descending (can’t flip it), some SHIFT controls would have helped (e.g. shift+blue could control direction of envelope), feels random at times
Hope that helps! Def worth the download IMO