GTalk was a rather nice XMPP implementation, it lacked some features like message archive, storing chat logs right in gmail instead (but then again, at the time there wasn't a good XEP for message archive).

When they have dropped Gtalk and went with Hangouts (likely, because of a WhatsApp envy), crippling XMPP compatibility one step at a time, they have claimed the reason for it is that it is 'impossible' to achieve great chat performance with XMPP. Of course, that was a lie. It is quite possible, and we're doing just that.