Personally, if anything, it makes me concentrate more on situational awareness. You're not busy on micro adjustments of speed, steering but looking at the road ahead, your mirrors etc.
Long journeys using adaptive cruise + lane keep are much less tiring.
Worth noting that Tesla's driven with Autopilot engaged (just their fancy name for adaptive cruise with steering) experience one crash in every 4.19 million miles in the US. On average, other cars experience a crash in every 484k miles.