In practice it feels like working with three libraries in a trenchcoat. However, as of five years ago it was totally worth it: the killer app for Sage was that

In practice it feels like working with three libraries in a trenchcoat. However, as of five years ago it was totally worth it: the killer app for Sage was that it let you describe a problem in SymPy where you have the full power of python for loops, library support, visualization etc; and then solve it with Maxima's more powerful symbolic calculus tools. I'm not up to speed on whether SymPy has caught up to Maxima in terms of being able to solve integrals, differential equations, etc.