Not sure how accurate it is but the fact that it's so reproducibly different does make me trust it a little bit. Also jives with how tired I am the days after I drink. The resting heart rate seems to be quite accurate, I've compared it to manually checking my pulse. During exercise it tends to be slightly less accurate because of the sweat and movement.

There is a Xiaomi product that supposedly can do this: https://www.mi.com/en/miband/