Didn't try downloading the SDK yet, but in the playground, seems like the "Python superset" advertisement does not hold (trying to define a class gives "error: classes not implemented yet"). Great to see progress in this space but I think the real killer feature of Mojo would be to run any existing Python program and allow programmers to incrementally adopt Mojo's syntax for targeted optimizations, and seems like we'…
Something feels so... off about this. It's a closed source product (I think? I see no where to contribute) so I can imagine the need for walling off local dev tools until they're ready for prime time, but it doesn't even seem like they are ready. They'll have you sign up before downloading some blackbox SDK that half-implements a language that they pinky-promise they won't abandon and you have no recourse to recover if they do.
If their intent is to flesh out a basic product then open things up, then I take that back, but as of right now, they seem to be pitching it as "ready to go" based on the copy here. That's a lot of trust to put with them.