I've been playing with React a bit to see how it feels. The new Camlistore UI is written using it and it's beautiful.
My first demo is Tic-Tac-Toe (code is here):
My second demo is a silly drawing tool that load-tests the framework with 40,000 child components (code is here). This one is kind of slow (discussion of its performance here):
I find the experience of creating new components in React to be extremely easy. It's liberating compared to Closure library's heavy-weight component model. That said, I've already hit a few cases where I was accessing non-existent properties and would like the advantage of the Closure compiler. It looks like this is possible but I haven't tried it.