Have you seen any of the hundreds of media reports, videos, or other threads on this forum? They all clearly show that there is no nag, and the only time you need to grab the wheel in the Model S is when you're about to die if you do nothing.
There is most definitely nag. Not quite sure what triggers it... my guess at this point from what I have seen is a large curve that the car has to navigate (even though it has no trouble doing so) will prompt it to nag me to touch the wheel. If I refuse, I get I think it is 2 more chances/nags... about 30 seconds apart. After that, the TACC will stop functioning (the icon still shows it enabled), the car will beep constantly, and a message will say something similar to "take control to resume speed". I drove a good 2 minutes in this state. The beeping is quite annoying. The car kept steering, but I had to modulate the accelerator as I would without TACC. After ~2 minutes, when i got tired of the beeping (and it may have been less than 2 minutes, but it felt like a long while), I gave the steering wheel some pressure/torque, and everything was nice and happy again. This same situation happened twice on large curved sections of highway, with a car in front of me. I do not think it was because the car was having difficulty, rather I think it is a programming decision that on certain size bends, the driver is then asked to respond. For what reason, I'm not sure (given that I could then drive 20 minutes without a nag, so it isn't to keep my attention).
Also, while i get the car wanting to in some fashion get the drivers attention, I am surprised that using the accelerator pedal and adjusting cruise speed via the stalk doesn't signal to the car "ok, driver isn't dead, carry on". I'm sure this will all be tweaked over time.