I've done a few thousand miles with Tesla's Autopilot and I find it to be much easier to stay awake with it than without. Constantly making little adjustments is pretty tiring. Paying attention to the the road and other cars is enough to keep my attention. Your experience may, of course, vary.
I don't know how long your durations on the wheel are - but microsleep[1] is notoriously hard to self-diagnose, even for drivers of non-self-driving cars.
It goes along with overall fatigue, right? I feel a lot less tired after a lot of Autopilot driving than after a similar amount of non-Autopilot driving.
As for durations at the wheel, I did a Virginia to Wisconsin round-trip over the holidays, and have done other trips of similar lengths in the past. Of course, I'm not spending more than about three hours at the wheel at a time, since the car needs to charge and I have various physiological needs.
