I have used ZeroMQ with C, Dlang, and Python -- mostly for learning.
However, I have used NetMQ.. a C# implementation of ZeroMQ in live software and the results are very positive!
I used a Pub-Sub pattern for one program to keep users informed on progress of a task, which could have taken hours to complete. They had a GUI program which spits out updates. It worked really well.
I was also tasked updating a Till software, which the integration of orders to the central system was extremely slow. I used NetMQ which was looking extremely successful but was put on hold due to IT manager not understanding Software Developers -- if something starts taking more than a week to do (which I stated I needed a month) they get itchy and move onto to something else. Sadly, that never got completed.
Now, I have played with NNG and there are some interested articles (or hidden pages from memory) about companring NNG to ZeroMQ - it seems the "patterns" are simplified.
I am currently in the progress of creating bindings for NNG. Seems to be pretty good, so far. I plan to move away from NetMQ (C#) in favour of this language moving forward.
Whether you use ZeroMQ or NNG - I dont think you can go wrong. It is all about the process more than anything, ensuring you do not lose data.
In 2014, I was tasked with rebuilding an event processing engine to increase throughput and performance. Used ZeroMQ with C# and also had a very positive experience.
It was very easy to build a multi-node, distributed event processing engine (think Apache Flink) that could scale by simply adding more nodes or threads. ZMQ makes coordination and management of messages easy and low-fanfare.
In our use case, it was stable and it was the least problematic part of a relatively complex platform.
nng looks promising, but the guide from zmq seemed like a killer feature. It describes all sorts of high level patterns, gotchas, etc.
For nng I mostly found API documentation, which made me a bit more cautious (though to be fair, I've not tried it yet).