The only reason to use polymer is if you need the L style UIs they discussed at Google I/O.

While Google's really good at Java apis, they're really terrible at JS apis. I love Google, but I would be very hesitant to use their JavaScript libraries or frameworks. A lot of people I've talked to (on irc, es-discus, etc.) don't agree with web components, but Google's pushing it hard.

Web components are poorly based good ideas, and polymer is poorly based on web components, and your app will be (poorly) based on that.

Other people have started posting some of the problems with polymer.