Flexible and high-performant so it was easy to adapt it to our distributed system needs It was also

Python library that u provide is easy to implement

I like the ZMQ concept of being "lego" of various socket type, which could be connected to each other, e.g Publisher - Subscriber, Router - Dealer, Request - Reply etc. For example when I needed to provide distributed logging my application I just used Publisher - Subscriber socket types, so multiple publishers sent their logs to one subscriber which stored them. And Router - Dealer model is great to send messages to specific clients and get response from them when the work is done. ZMQ allows socket sharing for threads and processes as well, which provides ability do build multithreaded or multiprocess application. Also ZMQ API is available for any modern programming language so can be easily installed and integrated. And of course, speed. I tested my ZMQ application under intensive stress on 40 machines and it keep doing pretty well, no stuck or lost messages and no crashes, so when it comets to reliability ZMQ is the right choice.