I am using Script # and have previously used GWT. They are really two different things. GWT is designed to provide a client and server solution with RPC and everything else. It is definitely more mature and you can switch to complex applications faster. Simply put, in the wild there is much more code and examples.
However, I think that if you are developers on both the server side and the client side, using two different languages and two different platforms can be very, very burdensome. This is why I moved to Script #. All I do is in C # and in Visual Studio, this allows me to be more productive. If you are not taking advantage of the GWT backend, this is really overkill.
I like to think of Script # as Javascript written in the C # 2.0 specification (which is). This is a fully client-side interface, and any mapping must be done manually (although automatic use can be widely used). This is very complete in support of Javascript and jQuery, in fact it is so complete that it surprised me at first. He seemed to be doing less than it was.
HenriDale's points are valid, especially with regard to community and openness. Although it was a bit of a thorn on my side, I really enjoy using Script #. I do not need to change the IDE, I do not need to look for how to do something in Java, etc. JQuery has a huge library of plugins, and it’s very easy to connect them to Script #. You simply drop a few objects to represent the properties, annotate them as "imported", and return null. In your code, you throw the object as a plugin, and your result is exactly the same as in Javascript. Script # does not care / does not know how the plugin works.
Do not let Script # a lack of community support fool you. Although this is a problem, the product is very mature and multifunctional. If your developers use C # / VS, why would they use a separate client program? I found that it was a huge success.
As an aside, I have become much better in Javascript since I used C #. Many problems with Javascript are the lack of language features that you really don't need, but in large projects this is the only way to make it manageable.