I think it's unfortunate that this is currently the top comment on this post. Someone puts hundreds of commits of work into a nifty language with a nice feature set (including compile-to-js) and awesome integrations, and this is what they get on HN...
I think this looks like a great project, we can never have too many programming languages to play around with.
With what crystal ball? If you pick based on history, since you kinda have to, then you pick a more mature (older) language. If you're not careful with that you pick languages just before they go obsolete.
(Like a company I know which picked VB 6 and still hasn't fully migrated away from it. The same company picked Microsoft's AJAX demo as a basis for a JS framework and is still developing that even though MS long since abandoned it. Would it be better to use jQuery, Angular, Ember, React, Riot, etc? Well, those are too new and untested. Very conservative leadership.)
Anyway CoffeeScript and JavaScript work well together and you might want your code to be in CoffeeScript but you have to use a library, say Ember (also a real project at a different company) which means certain improvements, bug fixes, etc. to Ember are done in JS. There's two. I'm sure you could easily end up with multiple libraries written in multiple compile-to-JS languages.
Long-term code maintainability is a bit of a hard problem. You probably need to be constantly refactoring, rewriting, and re-inventing so you don't have too much old code in play anyway. Maybe. What do I know?
I think this looks like a great project, we can never have too many programming languages to play around with.