I've had one since October and really like it.

I justify the ~$20/mo the same way I justify a gym membership: it's a bargain if it's compelling you to make positive choices about health/sleep/exercise. If it's not, then yeah, it's just an expensive mood ring.

Of course, there are also smart rings (Oura) and much cheaper devices (FitBit https://store.google.com/category/trackers).