Do you use maxima? Would you say it's still worth using and learning?

It depends of what you need, but for example for calculus is a nice program. There is also sympy and Wolfram Mathematica. For symbolic computation I think that Mathematica is the strongest then maxima and then sympy, but sympy is based on python and I think it will get stronger. If you need numerical computation then there is octave or matlab or julia.

For symbolic computation there is also Maple (*shudders at the thought*).

Yes, I think it is worth learning. GNU units is worth learning, too. You know these math riddles? You can solve it using GNU units alone. :D

Qalc from libqalculate it's a mini-maxima.