Polymer (and the Web Components) is a very interesting framework for composing web apps. The little I played with the framework gave me a great insight of its capabilities, especially due to the encapsulation of the Components.
I am still wondering about the upcoming browser compatibility of the framework. Currently, it’s limited, and I imagine it ought to become widely supported.
Edit : Currently, the support is natively limited, but the Polyfill library helps bridging the gap for all Evergreen browsers. The exception exists for Internet Explorer, which is correctly supported since version 11. That means no support for IE in Windows before Windows 7.