I am hoping that it is based around the standard HTML Push API but just with further OS integration. That API is supported by Firefox (and probably other browsers too) so would be useful so get widespread support from websites.
It's not.
I doubt many websites will support it if it is only useful for people using Safari on the very latest version of OS X as they would make up a small proportion of all visitors.
It does sound like a pretty hard sell, given the need for an Apple developer account and the restrictions that Apple will impose on the feature's usage.