I did a road trip this weekend and used autopilot for more than 400 miles on a brand new P3D (with the FSD option, limited though it is right now). Mostly freeway, some rural roads. Here are my observations, I assume it's pretty typical but obviously all I have is anecdata.
Lane following works effectively, but it wanders a bit. Mostly not enough to be bothersome, though sometimes it will hug one side or the other close enough to make me think about the big truck next to me. Sometimes a little late reacting to a curve, though not to the point of going across the line. Planned lane changes work pretty nicely. Identification of vehicles around the car is good towards the front and noticeably spotty to the rear quadrants. It reliably misidentifies the merging traffic lane as one big lane as soon as the line ends, and promptly drives to the right to straddle the 'middle' and then corrects as the line converges to the normal width.
Adaptive cruise control is flat out disappointing. Granted, I've never driven anyone else's automation, but it actually made me motion sick at times. I thought it did a really admirable job of handling people merging right in front of the nose of my car without jamming the brakes. Just smoothly slipped back until the right following distance was achieved. But in open traffic, when I came up on another car, it would abruptly slow down a bit too aggressively, and then fiddle with the speed a bit until it matched the leading car. If the lead car switched lanes, it waited until several seconds after the car was in the next lane and then abruptly sped up to the programmed max speed. There were times I could tell how frustrating this style was to people behind us who were not using a similar system.
Twice it braked suddenly, though not any kind of panic stop, and then immediately sped back up. Like it saw something concerning and then changed it's mind a split second later. Made my wife a little nervous after the second time, but it didn't happen again the rest of our trip.
Overall I'd classify it as a moderately drunk driver :). I let it drive us into a couple moderately uncomfortable merge situations just to see how it would deal with traffic entering the freeway, and while it didn't get us in a wreck or anything, it didn't make me feel like AP is on the verge of becoming viable FSD.
For what it's worth, this was on I-5 in between Portland and Seattle, in mostly light traffic. We have what are, in my experience, pretty easy relaxed freeway configurations. Long ramps, no ramp lights, etc.
I borrowed the P3D so I could have myself an extended test drive to decide if I really wanted a Model 3 or not. Wicked fast (which is still an understatement), I love the power but would probably choose to steer it myself for the forseable future. I wish they'd put in nicer seats (like, my legs were getting numb at one point!), just standard Recaros like everyone else would be perfectly nice. I think I've decided that I really like it but I'm going to wait for Model Y before pulling the trigger. I will appreciate the additional head space and seat height, and hoping that in the meantime they do v3 or v4 of the seats.