The blood oxygen level has been useful to me. I have both asthma and anxiety. Figuring out what’s an asthma attack and what’s a panic attack is helpful.

Have you found one that's actually anything like accurate? Every smartwatch I've tried either reads way low or has some other ~obvious sign of not being correct.

My Apple Watch seems accurate enough, if the pulseox reader I have is correct.

How about being served ads based on your state ? Is this still ok ?

I mean it's pretty useful information. Imagine seeing an indicator that tells you that you have elevated levels of stress, and then you can try different things to reduce the stress. Eventually you could learn, with pretty decent confidence, what works for you and what doesn't.