Speaking as someone who uses Mavericks:
1) Its a great feature. Of course some websites will try to spam, but you can just turn them off. Super simple too.
Examples: Gmail, Weather, etc.. It removes the need for native apps whose sole purpose is to poll a website and provide a notification.
2) Its completely opt-in per website.
3) You can turn it off wholesale in Safari, or per website.
So those of you who say stuff like "first time I get one, I'll stop using...