I started Sage, and worked on it a huge amount 2004-2012, but less so recently (due to working mostly on CoCalc). Actual contributions to and work on Sage by the community have gone UP a lot since 2012, as this plot (https://github.com/sagemath/sage/graphs/contributors) at Github shows. Much of that work is from research mathematicians that are adding large swaths of important functionality (e.g., in differential manifolds) to Sage. But there's also a very welcome push for quality, e.g., the number one recent contributor, fchapoton, has been fixing every little detail in the codebase revealed by https://lgtm.com/ and other tools, and there's also been a lot of work to fully transition Sage to Python 3 (which is difficult since so much is written in Cython and make nontrivial use of comparisons).

I would have likely made Sage pip-installable from day 1 if I had known that pip would turn out to be so popular. It was really unclear what was going on with Python packaging 15 years ago. I'm really glad that one package manager (pip) has become so good at this point.

I use it for various all-purpose calculations. It works well for me.

Sure; sage is a mainstay amongst cryptography engineers.