For the moment, it doesn't really have great browser support and less intent to implement than some other features of the OWP. Whereas service workers at least had Mozilla pushing along with it, Google seems to be "going it alone" a bit more on Polymer and having a tougher time building a coalition for it.
Hmm, I've found that the polyfills work fine, if you need to target IE11 and latest Firefox/Chrome. Even old Chrome versions work well with the polyfill.
The shadow DOM polyfill works fine in IE. There are a few problems with IE though, the main one I've encountered is that dom-repeat doesn't work inside table.
