I find GWT to be the best platform to do full-blown fat client-side paradigm due to several reasons:
1) UI Templating (like XAML)
2) MVP Pattern + JUnit = test automation as part of your build with minimum investment to infrastructure (most JS based solution requires investment on infrastructure)
3) Resource Bundle (similar to Asset Pipelining) cut down HTTP requests
4) Resource splitting (follow up from #3) to reduce the size of the responses when it comes to resources
5) JS binding (like C binding, C++ binding, JNI, etc) to support popular JS library
6) Top Notch Compiler (arguable... but it's there) that can also prune dead code
7) JS switching based on browser's request (IE browser will get IE-specific JS)
8) CSS variable substitution (like Sass)
9) History framework.
10) Flexibility: end point can either return JSON, XML, or use the built-in XML-RPC mechanism (Java only).
The disadvantages are: it's Java and requires compilation.
I like GWT the most so far because I don't have to find libraries or tools that support all of the above (Sass, mustache, various js unit-testing tools, library to merge and compile css/js, etc).
I like GWT as well, but its development history makes me a bit nervous about its future.
It is open source, but doesn't have a developer ecosystem outside of google employees (in fact they outright state that no one else can be a committer). That in itself wouldn't be the end of the world, but unfortunately google has a) reduced resources devoted to GWT dramatically in the last year or so (many team members were reassigned to the dart project), and b) has a history rapidly changing 'paradigms' and semi-abandoning the code and documentation for the old way of doing things.
I think the biggest danger of using GWT is when it breaks on newer browsers and this has happened once in the past with the release of Safari 4 (or 5?) there was a minor hiccup in which they quickly fix it the next day.
That is definitely a major glare. Other than that, if the community decided to fork it (and have the time and capability to do so) it'll be a safe choice for a long time.
GWT as of today is definitely very polished and well maintained and fulfil the needs of Single Page App smoother than having to hunt different tools and libraries and to set them individually to be a part of the build tool.
1) UI Templating (like XAML)
2) MVP Pattern + JUnit = test automation as part of your build with minimum investment to infrastructure (most JS based solution requires investment on infrastructure)
3) Resource Bundle (similar to Asset Pipelining) cut down HTTP requests
4) Resource splitting (follow up from #3) to reduce the size of the responses when it comes to resources
5) JS binding (like C binding, C++ binding, JNI, etc) to support popular JS library
6) Top Notch Compiler (arguable... but it's there) that can also prune dead code
7) JS switching based on browser's request (IE browser will get IE-specific JS)
8) CSS variable substitution (like Sass)
9) History framework.
10) Flexibility: end point can either return JSON, XML, or use the built-in XML-RPC mechanism (Java only).
The disadvantages are: it's Java and requires compilation.
I like GWT the most so far because I don't have to find libraries or tools that support all of the above (Sass, mustache, various js unit-testing tools, library to merge and compile css/js, etc).
YMMV.