When the acrylic gouache first came out, I was sure that it was a terrible idea - until one of my friends recommended it.
Having tried it, I do think it has a place - addressing envelopes.
However -- DO NOT MIX IT WITH REGULAR GOUACHE.
It does not reconstitute.
I would never use it for my own artwork, but it streamlined some of my envelope jobs which was fantastic at the time.
Here are the features that I really like:
1
It is not entirely waterproof, but it is very water resistant, so it is a good choice for envelopes.
2
If you thin it enough, it works fine with a Z type nib.
You will not get the extreme thicks and thins, but it works fine - for envelopes.
3
If you don't want to bother mixing a particular color - you can usually find something that works which saves time.
4
If you do a lot of envelope addressing, at some point, you will get a job with a problem paper
and in my experience - it works well on any kind of paper.
I prefer the stiffer Z nibs for addressing and I am not interested in the more delicate nibs and the thicker thicks.
So, I can't say whether the acrylic gouache works well with other kinds of nibs and paper combinations.
It is not as dreamy as regular gouache, but, when it comes to envelope jobs, I am happy to have it as an option.
I never found any of the acrylic inks that felt as good as regular gouache, so, to me -- it's the same difference as acrylic ink.
I agree about buying only good quality gouache. Do not buy cheap gouache.