First things first: I like autopilot and use it whenever possible. But I've started to wonder, whether it is actually usefull. I think it does not reduce my work, because I have to be alert all the time just like without AP. Of course this is just one step in making fully autonomous car and I appreciate this, but I don't think it is particularly usefull at the moment.
Presumably TACC alone is generally considered very useful as it's the next move forward from basic cruise control. Now you don't have to be constantly adjusting your speed with your cruise control lever. I often used to "drive" with that damn cruise control lever to throttle my speed. On those cars without adaptive cruise control I used to find myself mindlessly doing something silly like lowering the volume (on my steering wheel) when I needed to slow down because the volume buttons were opposite the cruise control. My point is, removing the element of speed control from the cruise function made the whole cruise control experience infinitely better and safer. Set it and forget it (to an extent).
Similarly, AP/Autosteer is a next move forward for TACC. Now you're handing off the steering to the vehicle and reducing the number of needed inputs to the system. Again, it makes things that much easier...and useful, to answer your original question.
That's my opinion at least. I haven't had a chance to use AP on a really long road trip yet, but I find it incredibly useful in heavy freeway traffic (stop and go) in particular. I can relax a little bit more, enjoy my music and let the car do the majority of the work. I'm always watching the road, and my hand is resting on my left knee right by the steering wheel ready to intervene at any moment. But it's still so much more relaxing not having the worry about the constant pedal work and stopping/starting of that stop and go traffic. I don't have to worry as much about rear ending a car in front of me because I happened to glance away for that 1/10th second to change a radio station, the same moment when he decides to smash on his brakes. AP is always watching. And so in that instance AP is very, very useful and greatly reduces the amount of work/input needed from me.
All that said, if I'm using AP on a winding road at higher speeds with crappy lane markings and moderate traffic, AP is definitely less useful because I am having to constantly babysit the system MUCH more than in the above scenario. And that's why I don't really use it in this instance (nor does Tesla even recommend that). It can actually make driving more stressful because I'm just waiting for AP to disengage at some dangerous or inconvenient moment.
As far as "having to be alert all the time," that's a mandatory condition for getting behind the wheel at any time, regardless of the tech your vehicle has in it. We won't be able to have a discussion about NOT being alert until the day we can get into our car, punch in a destination and your vehicle can safely take you there with ZERO inputs while you sleep the entire drive.
There are definitely varying opinions on AP and the above is just one perspective. I could see if you only had the opportunity to use AP in my winding road example, that AP might seem less than useful. But it's definitely not a gimmick!