I've used it a couple of times and intend using it again starting in a couple of weeks, for a very specific purpose (which is to have a Q&A session a couple of times a week).
It went ok, until I tried doing it outside at which point my internet connection just wasn't strong enough and people got very irritated, understandably, because it kept pixelating or dropping altogether. So making sure of a good connection is critical, I think.
The other problem I had was trying to keep up with comments and not look like an idiot. Doing a live video whilst trying to see what people are writing is hard going, or at least I found it so. Squinting at the screen is not a good look. But it's really important to acknowledge what the audience is saying, so a way has to be found. The "gurus" of social media tend to have someone else reading any questions that come in aloud for them and I intend to rope my husband in when the time comes.
In terms of whether people automatically see it, no they don't, as far as I'm aware. You have to advertise it fairly widely. The only way people will see it automatically is if they have turned on "notifications" before it starts. The info about how to tell people to do that is at this link
I've seen a lot of big social media people (like Marie Smith for example) trail their live session a few times - and at different times of day, to catch different time zones - before the date and get people to turn notifications on. It's always worth taking a look at some big names, either general "experts" or people in your specific niche with a large following - to see how they're making best use of it.
One other thing I'd say is during the session, make sure you tell people to "like" your FB page and tell them how. You're likely to pick up some random people as you go, who have not yet liked your page - or at least, I did.
I do think it's worth doing, and the analytics FB gives are good for analysing what's working and what's not. For me, people's attention span was around that of a gnat, so my sessions will be no longer than 5 mins or so. FB has changed how long a session can be so you can now go live for up to 4 hours. I wouldn't recommend that!
Hope that helps.