Sage is fantastic! I used it for math research for years; it's python, and brings together many of the best open source math projects. So yeah, there are seams, but the seams are far less bad than (say) writing your own translation layer between the various components. And Sage has a large (and always growing) base of native functionality, as well.

In fact, I'm pretty confused as to why someone would go to the trouble of trying to write a new, python-based mathematica alternative with Sage already quite established and equally free. (I guess keeping the Mathematica language is a motivation for someone who's tied to the mathematica world already, but I've always found it to be a write-only language...)

Yes bug Sage is weird. It's a sewed together giant of multiple parts, and the seams are quite apparent. It's also hard to install and get started with.