<p>Right now I'm not sure...</p>

I'd say yes.  I'm using it.  I know for a fact that Universal are using it on some of their (thousands of) sites.  I will add some caveats, however:
There are serious problems with setting it up, especially if you want to debug into the libraries.
The helper functions favour prototype, as opposed to the more modern jQuery.  This is changing rapidly, however.
The documentation is a bit chaotic, again the Castle Team are working on that.
I'm not guaranteeing every last "out-there" feature works, but the point of the system is actually to keep it simple.
Compared to vanilla ASP.NET, it's an absolute joy.  I assure you that you won't miss viewstate.