It’s exciting to be able to deploy a true “finalized” responsive image spec to the public, something in upcoming months that should be rapidly adopted by the browsers. That’s exactly what happened to us at Pocket – a stable version 2.0 release came out a few weeks before I needed it for our latest major release. If you’re already using some other, competing responsive image solution, I’d recommend looking into Picturefill and consider switching over. It’s a polyfill that’s very well equipped for the finalized picture element.
The implementation was pretty painless, though I did find occasional image pop-in when using the srcset attribute in a solo image (no picture element). There’s also sometimes a double image download in already native-supporting srcset browsers (e.g. latest Chrome). It’s a known temporary issue and only a minor downside given Picturefill’s overall benefits.