On long x-countries, and especially in IFR, I fly AP. The one in the
172SP from the flight school usually works well, tho it has been known
to act up - inducing a roll for no apparent reason. So I always keep
an eye on the AH and TC to make sure nothing untoward is going on.

East coast flight, like another poster said, especially single pilot,
AP comes in very handy. I fly in and out of NYC airspace, and coming
in during a heavy push, single pilot IFR, I don't care if it's CAVU,
the AP is on the whole time til I'm cleared for the approach. You
don't have time to be distracted by anything when the freq is that
jammed up, and unless it's an emergency, ATC doesn't want you doing
anything they're not expecting... otherwise expect to get booted out
of the Bravo pronto.