Right now I'm not sure...
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.
We have been building a fairly large application with it for the past year and a half. Its been nice not to have to deal with the old ASP/Page based model and use the better Model/View/Controller design pattern.
To get the new stuff you really need to work off the trunk of development because they don't do releases very often. We have a lot of tests that get the framework involved so when an update in the framework breaks something we depend on we know about it immediately.
If you have to work in .NET this beats the heck out of the alternatives.
There is an overview on the monorail forum: http://forum.castleproject.org/viewforum.php?f=6
I'm using it for an application and haven't had any big issues with it. The biggest problem is indeed find good documentation and examples.
I've had no problems setting it up. Julian, I don't think it is constructive to say things like "serious problems" without any further clarification or example.
Debugging into the libraries is trivial. Because it's open source, you can debug into the whole thing.
I've been using MonoRail for production for ages on many projects, as an employee, as an indie contractor, and for non-work related sites.
I know I'm biased on that, however I can whole heartedly promise that my positive usage experience is what lured me into contributing to the project, not the other way around.