[piannucci]

I use it from time to time for work. It lacks some of Mathematica’s typographic flair (can’t ctrl-^ to get a superscript box or / to get a fraction) but IIRC has better undo support on desktop and satisfactory performance as long as your output expressions don’t get too big. The web layout engine chokes pretty badly for medium-to-large expressions, although there is an automatic transition to pre-rendered images at a certain level of complexity.

I use it when I want an industrial-grade CAS for some nasty integral or when I want to do something LISPy.