I saw the Reason announcement months and months ago and decided to use it as a jumping-off point into the OCaml ecosystem. It took about a week of writing Reason before I realized I was spending so much time reading OCaml (all the libraries were written in it) before I just switched to OCaml proper. Haven't looked back - the syntax takes a few days to pick up, but is hardly the most difficult part of writing a program.
Hopefully they can capitalize on the "Build" and "Share" part of their value prop [0] because "Syntax" appears (to most) to be the least valuable improvement. As a disclaimer, I could be totally wrong, never having worked at a FB-size organization – that onboarding time probably adds up quickly.
Come to think of it: can anyone think of a successful transpiled language that doesn't actually provide new semantics? Coffeescript is the closest thing that comes to mind, but it's dying out as ECMAScript absorbs its most appealing features.
[0]: http://facebook.github.io/reason/#reason
Edit: forgot about Elixir, which seems to be doing quite well – but that introduces new semantics like homoiconicity + macros and protocols.