That's hard to say until they actually release documentation. When I first noticed it [1] 10 months ago, it seemed very flaky and slow, but it appears to have improved a lot in the meantime.

[1]: https://twitter.com/marijnjh/status/252814288891285505

Still, it's closed source, so for now, not very relevant for the rest of us.

So, no tern.js for them until it's open source :)

I'd love to know their motivation for writing their own instead of contributing to Ace or CodeMirror.

I freaking love c9. But I will try out the MS thing.