A bit random
Most of the time it's okay. Sometimes it turns itself off for some reason (with plenty of battery left) and I can't figure out why or when. Sometimes it gets stuck in a state where it refuses to recognize that I'm bent - I can look at the app and see the red arrow going under the green arrow, indicating bad posture, and the buzz-timer starts counting down from 15, and then randomly it will "jitter" back to perfect posture and back to bad posture. This causes the timer to reset and the Upright to.. not buzz when I have bad posture.
These occurrences seem to be rare, but they're confusing and making me lost trust in the device.