OFF for me as it never adjusts the way I want it anyways. However, you might need to also turn-off other items that use the light sensor (eg white balance), to realize any power savings (beyond dim screens). That said it’s probably one of those insignificant features that’s a mouse fart by itself… but add up dozens of mouse farts and you get a big turd.
Advice from one of the very few folks that actually gets Apple’s ‘up to’ SOT specs - there’s a lot more low hanging fruit out there (eg, Apple using your phone as a crowdsource beacon). You just need go though the settings, and evaluate what’s worth it for YOU: a given feature vs batt efficiency.