I watch this project since a few years and they make good progress. To whoever is interested in open source Computer Algebra Systems, there are of course plenty of more mature solutions. Classical ones such as GNU Octave or Maxima but also "modern ones" such as SAGEmath, Symbolics.jl or sympy. In particular, there is a broad range from symbolic libraries such as GiNaC up to "battery included" IDEs like SAGEmath. The community is vivid and amazing, for instance SAGEmath basically pioneered the web notebook interface which today brought us Jupyter in all its fashions.
I personally love the LISPy style of Mathematica (MMA) but of course it is not the (only) the core which makes MMA so powerful but the super large library which has not only instance industry-leading solutions for basic topics such as symbolic integration, 2D/3D graphics or finite element methods but also a plethora of special purpose domains such as bioinformatics. I guess Mathics has a good clone of the core but lacks, of course, all the libraries. It is, by the way, the same logic as with Matlab and its many "toolkits" compared to the numpy clone. However, the python movement brought many novel codes into the numpy world which no more work on Matlab.
I personally love the LISPy style of Mathematica (MMA) but of course it is not the (only) the core which makes MMA so powerful but the super large library which has not only instance industry-leading solutions for basic topics such as symbolic integration, 2D/3D graphics or finite element methods but also a plethora of special purpose domains such as bioinformatics. I guess Mathics has a good clone of the core but lacks, of course, all the libraries. It is, by the way, the same logic as with Matlab and its many "toolkits" compared to the numpy clone. However, the python movement brought many novel codes into the numpy world which no more work on Matlab.