I just got it this morning...
My very first impressions were a bit mixed, it at first seemed a bit hard to control but once you understand that most of the controls are pretty sensitive to the other parameters, it starts to make sense. Diffuse is basically the attack of the amplitude envelope of the reverb, drag is used for both pre-delay and some custom stuff it seems.
I read in an interview that this pedal is basically the guy from Earthquaker messing with some reverb code without being too sure about what he was doing and ending up with a happy accident, I wish I could check the code and tweak it a little bit to make the pre-delay more independent of what's used later to handle the repeats

.
It's actually possible to get a simple enough reverb (indeed not room reverb but not too cavernous either) with minimum length, max diffuse, minimum damping (i.e. low pass filter at that setting), max drag, min reflect and just a bit of reverb in the mix.
So far I like it but it's definitely not as immediate as most reverbs. And it has some limitations, for example you can't completely reduce the predelay to 0.
One of my only complaints is that it does not have a lot of headroom. All my Boss pedals (and my mixer) and even my crappy Behringer reverb handle signals from my modular with just a bit of attenuation (or none at all sometimes), but the Afterneath definitely does not like hot signals. Line level or so is ok but not more or it starts clipping and not in a nice way... It also seems to slightly boost the volume even with the mix at 0. And it's sometimes hard to stop the last little bits of reverb trails even with low reflect. You sometimes have to reduce the length to get the algorithm to stop regenerating the reverb.
Anyway, I'll have to spend more time with this one. Lots of potential but you have to earn it. Seems more "forgiving" with guitar than synths, too (i.e. it's easier to make something that sounds good on guitar).