Closure has been available for a while and is being used with Maps, Gmail, Docs, Reader etc. so I don't see why the addition of another Google product would make a big difference in adoption.
It is a large, full-featured framework, comparable to sproutcore, cappuccino or EXT. It ties in a lot with Java and (update: not GWT: I must have missed this since I never used GWT). I tried it, but found that I prefer a lighter stack of loosely coupled libraries (eg. backbone + mustache + zepto)
Actually it doesn't tie in with GWT at all. It's kind of weird - there doesn't seem to be anyway at all to use the Closure components from GWT without a lot of work.
Closure has it's own Javascript compiler (that does some type checking and minimisation). It's likely that is what you are thinking of - you'd hardly be the first.
it comes down to understanding everything you are using, and being able to swap out components for something better. ie. it is very unlikely that a single framework has the best router, controller, selector, templating etc. but by assembling your own stack you can inject personal preference and what you consider 'best'.
I do the same thing on the server side as opposed to using Django
It is a large, full-featured framework, comparable to sproutcore, cappuccino or EXT. It ties in a lot with Java and (update: not GWT: I must have missed this since I never used GWT). I tried it, but found that I prefer a lighter stack of loosely coupled libraries (eg. backbone + mustache + zepto)