http://www.polymer-project.org/resources/compatibility.html Even for latest Chrome half of the stuff isn't supported natively yet - performance with polyfills is quite bad from what I read. It's even worse for other browsers. See discussion here: https://news.ycombinator.com/item?id=7971030 IE 8 & 9 have 8-10% market share. IE10 support is still flaky. Remember, that most sites (not apps) didn't drop IE6 support until it had <1% market share.
tl;dr; It looks more like a demo of bleeding edge web tech, but not something that you can use yet.