I have used EnTT for a few half finished projects. I like it because, as a more intermediate programmer, it forced me into the pattern. Sort of like coding on rails a bit in a good way.
Part of my feels that it is too much of a bother for most projects. Another part of me feels like that the ECS pattern is only going to get more common and that I should be concerned about data locality and speed no matter what (otherwise why am I even writing in C++?). I've honestly never had a project which needed the extra performance boost that an ECS library would give me. Usually my bottleneck is inefficient collision detection and noob rendering.
Two parts of my mind are conflicting: one tells me that using a library is great because it lets me how things are supposed to be done, and that I should use a library of every type before I try to roll my own. Another part of my mind tells me that I will never fully learn this stuff without rolling my own and that I am wasting my time using a library when I can't even roll my own and don't know completely how it all works.
Anyhow, let me know your thoughts and experiences with ECS libraries.