Another issue i've encountered is that there's no way to see the status of the request without having the XHR object.
i've made the following changes:
isSuccess: function(xhr) {
xhr = xhr || this.xhr;
var status = xhr.status || 0;
return !status || (status >= 200 && status < 300);
}
This way i can alway check the readystate of the xhr object.
I don't know if this should be in the defaults of polymer but the ajax and xhr objects definitly need some work..