Recs.
Updated
SpecsUpdate
Pros
Pro High-Performance
Compared to multiple Web/Web-UI frameworks, Polymer is really interesting. It is a framework built on top of new W3C standards, so building custom elements and reusing or sharing them is really easy.
Pro Complete web app stack support
Full app stack from data tier to routing, progressive web app, responsive layouts makes no need to seek outside of Polymer ecosystem for application features.
In addition to waste set of mature web components in Polymer Elements along with Vaadin Elements there are thousands of web components in the wild comparable to jQuery plugins set.
Pro Developer and user-friendly
The documentation is really good, it explains the basics as well as the advanced concepts.
Pro Excellent documentation
Polymer guides you as with tools (cli, build environment, app templates,..) as with complimentary documentation on all phases of app development from creation of app as progresive web app to production deployment instructions.
As Polymer is standards based, the whole community around those standards also helping in documentation and support.
Pro Based on web components
Web Components are a collection of specifications released by W3C as a way to reduce the complexity of web apps by creating reusable components. Browser support is currently poor for web components, however Polymer is developed to make web components compatible with modern browsers.
Cons
Con Has quirks that you need to be aware of
There are things that new developers need to be aware of when developing Polymer which aren't necessarily intuitive. For example, the way attributes are serialized into properties and the changing of properties from camel case to kebab case when they're initialized via attributes.
Other examples include the need to make sure Polymer is aware of object and array manipulations via explicit notification functions or by using provided array manipulation functions in created elements.