Joking aside this seems like a pretty useful idea.
The idea that you could provide pre-canned training where you can stop in the middle, rewind and poke about or experiment is compelling. Combine that with a safe sandbox that you can blow away at any point and you've got a winner.
Crucially though I think they need a really good security story before I think I'd use or recommend it myself though.
You can also roll your own tool, the format is reasonably well documented and simple.
> If the filename argument is omitted then (after asking for confirmation) the resulting asciicast is uploaded to asciinema-server (by default to asciinema.org), where it can be watched and shared.
If the filename argument is given then the resulting recording (called asciicast) is saved to a local file. It can later be replayed with asciinema play <filename> and/or uploaded to asciinema server with asciinema upload <filename>.
You can also optionally upload it to the service - if you want to.
> When you run asciinema rec in your terminal the recording starts, capturing all output that is being printed to your terminal while you’re issuing the shell commands. When the recording finishes (by hitting Ctrl-D or typing exit) then the captured output is uploaded to asciinema.org website and prepared for playback on the web.