I'm jealous of the people who have had good AP experiences. Mine have been at least 50% terrible ... err, I mean exciting. Very exciting to me and everyone around me, in a "hope we're not going to crash" kind of way.
The lane markings on the highways around here are not bad at all. They're actually very obvious and visible to humans. But they are clearly not what the car is looking for. What the car is looking for is paint. Solid, painted white stripes.
What we have here in Vegas are reflectors. Raised, rounded reflectors that we call turtles. A series of 4 of them takes the place of a single stripe, so the lane marking is a repeating series of 4 turtles, with about the same length and spacing as painted stripes.
In daylight the car flat out refuses to recognize the lanes at all. It wouldn't even go into autosteer on a well-marked freeway!
At night it's even worse, because the car sort of recognizes the reflectors, but it constantly drifts around in the lane as it adjusts where it thinks it should be. Twice the side impact warning went off due to excessive drift, and I'm not sure it would have stayed in the lane at all if I hadn't grabbed the wheel. I'm lucky I didn't get pulled over for a field sobriety test.
Tesla, please come do some testing in Vegas! It's not that far to go, and you'll really like it here. For now, AP is not usable on the highways I normally drive (215 and 95).
I guess the problem is similar to this one in Hawaii. Maybe you could do some testing there too!