I used it for a project about a year ago (https://github.com/STRML/securesha.re-client/tree/master/pol...). I found that it was difficult to keep the service up and working; while older browsers still seem to load it quite well, Chrome continues to natively implement parts of the (changing) spec.

When Polymer detects native capability for a given feature, it turns its polyfills off. But if the spec has changed, you are no longer supporting the spec as it was when you wrote your code; and inevitably, your application will crash.

I think Polymer has a lot of potential - I definitely enjoyed using it (check out the `app/elements/raw-xhr` and `webworker` elements for some fun; invisible DOM elements that process data!). But if I were to do another application like this, I think I'd just do it with React. It's less of a moving target and has a lot of the same benefits (and more), with the exception of CSS sandboxing.