I have it today too (6P, AT&T, off of Beta for now, so 7.1.1, March patch).
I didn't realize it either, no notifications, but when I held down the home button (per the above), there it was, sure enough.
It's not exactly obvious if you just swipe, as it looks fairly similar (to someone who doesn't use the GoogleNow page much) to the old interface. However, if you swipe and check settings, it's the "Google Assistant - Settings", no question.
I would bet a lot of people have it, and don't realize it, like myself.
Having done a fair amount of voice development work (both architecture/design and some coding and managing teams), you start to realize the scaling that must take place, to roll something like this out. One would assume Google would be rolling the same server resources from GoogleNow->GoogleAssistant, any other approach would be a bit boggling, in terms of the processing. Very close to 100% (there are usually a few exceptions, such as dialing, etc) of the processing takes place off-board (I'm sure most know this, sorry for the re-up, but I figured it was worth mentioning, to bring context).