99% of the time I'm on dual carriageway, or in stop-start queuing traffic. Off hand I can't think of cases when I wouldn't use it (in those situations) ...
I am totally vigilant though, just "differently" in that I am now able to scout further ahead, behind and around. I take the view that I don't care whether I, or AP, detect a problem - either of us is better than just me, and there are things that AP will react to faster than me - traffic intruding from left whilst I am looking in right wing mirror; car in front braking heavily but with broken brake lights (you'll get an alert for that which will wake the dead! but without AP you also have reaction time before you brake - or AEB cuts in, which may not happen until its time to "reduce the inevitable impact". Whereas AP would be on the brakes from the instant it detected that the closing distance was decreasing sharply.
Plenty of false alerts too - had one yesterday, person waking on a dog on the edge of the road. Dog was on a lead but piloting a course that weaved from side-to-side ... looked like a threat to AP (I presume it couldn't see the lead
) ... fair enough.
The risk of AP making a life-threatening decision may be as low as once-in-several-car-ownerships, which is so low as to create a complacency risk. I was a bit cavalier when I first got AP (2015) but a number of AP deaths (people watching movies / reading books / etc.) made headline news and I now assume that will happen at just the moment that I am doing something else - e.g. glancing at dashboard. There is no way I'm going to attempt a text-message assuming AP is not going to screw up at that moment. I've driven over 100,000 miles on AP and not had to disengage to avoid something totally unexpected, such as AP aiming for the concrete barrier where two lanes split, but I have intervened for situations like debris in the road and white-van-man has parked half-on-the-pavement.
From what I have read here some will say that AP tries to drive them off the road every time they used it. I (and others high mileage drivers here) have never had that ... despite a fair amount of discussion nothing has been raised that suggests why one persons experience can be so different.
Also some people have very frequent phantom braking. I (and others here) almost never have that. Also no idea why some-do and some-don't, but I can imagine that would be a PITA, particularly if there are any passengers in the car.
I find it more mentally taxing to use it as I'm always on edge waiting for it to do something stupid
In my case I find long journeys much less tiring on AP than before (which would have been just TACC in my case). No idea why removing the need to make small steering adjustments would be significantly less tiring, but that reduced driver workload definitely was for me. I had a number of regular long journeys thus easy to compare before/after getting my first Tesla/AP - one of them was a 9:30-11:00PM journey which, previously, I was always fighting tiredness the last few miles of dual carriageway - never had that subsequent to getting Tesla /AP.
I have AP set to max follow distance. On AP I never care if someone pushes in in front of me - that probably contributes to a reduction in stress / tiredness !