My 1 y.o. car has "self driving" features in it. My first car with any of that. The dealer said it is essentially the same as what Tesla calls autopilot (which is overstating it more than a bit, as I came to find out). It does 3 main things when you go into Full Assist mode.
1. Adaptive cruise. It allows you to adjust the number of car lengths to the car ahead of you within limits and it will maintain the speed until that distance is hit at which point it reduces speed to hold the distance. When the car in front is no longer there, it goes up to the set speed again. In Full Assist mode it can even come to a complete stop and then start going again when traffic backs up and restarts.
2. Lane keeping. It takes over the steering to a degree and holds the lane. You can let go of the steering wheel for only a few seconds before it yells at you and then disengages the auto assist if you don't grab it again. You can always manually override the auto steering just by pushing the wheel a bit harder where you want it to go.
3. Lane changing. It can maintain control of the vehicle and change a lane, but you have to tell it to do it. Say the cruise has slowed you down for another vehicle in front of you but the next lane is wide open. If it is clear, you flick the turn signal, and the car will move over and then resume the cruise speed. And if it does not like what it sees, it won't do it. If there is a car in the blind spot, for example, it won't move. The turn signal initiates the request to the system.
My daily commute is some country road and then 15 mi of freeway and I basically never use it on that as it is short and I like driving it (sport sedan). But on long road trips, it is very nice to have it take some of the mental load off of driving. You can't go to sleep or anything as once in a while it will kick off as something doesn't look right (bad lines or something) so you must remain aware, but it is nice to be able to open a bottle of pop without worrying about keeping in your lane, for example. And if you hit stop and go traffic, it is awesome! (Looking at you, Chicago)
A lot of these features are still running in the background too even if you don't have the Full Assist running. The lane keeping is kind of always on (unless you switch it off) so if you start to wander it will nudge you back in, but you can manually overcome it again. The emergency braking is also a great thing. I have never had it actually brake on me, but there have been a couple times where traffic suddenly slowed or stopped and I had glanced away for a sec. But it flashes this big red warning that you can't miss on the display which snaps you back to attention right quick and allowed plenty of time to slow down myself before it would kick in the auto stopping.
Overall I think it is a great thing to have, but given what I have seen from using it, the likelihood of full auto driving in the real world seems waaaaay far off, if it ever occurs. Simple example: winter with any snow or ice on the road - forget it, it won't work in full auto. There is just an infinite number of situations and variables to have to deal with, like the snow example. You would need some true AI that works like a human brain or better to make this happen.