Not to mention, that react side-steps the entire issue of heavy DOM reflows/rerendering as individual components update. I don't see Polymer doing anything but make that issue worse until browsers support it natively, and even then it will take some changes to the way browsers handle rendering to improve performance there.

I do think that long-term (5+ years) something like Polymer will become the preferred solution. For now, imho, React is about as good as it gets for componentized web applications.