Looks like an interesting halfway house between a text editor and a full-blown CAS like Mathematica (since I have a license, that's what I use as a scratchpad for this kind of thing). It also seems to overlap with Soulver, which is my go-to calculator on iOS.

Cleverly, Calca appears to go a bit further than even MMA by back-filling changed definitions, almost like functional reactive programming. This is neat and something I've wanted to see for a long time.

I'll be buying once I get home to my iPad, but I really want to see the Mac OS version!