In short, there are three complaints against the UP: (1) the device itself gets bricked and cannot be operated or synced to the iPhone at all; (2) the battery stops holding a decent charge; and (3) the removable cap which goes over the headphone charging plug is easy to lose.
Garrett Murray got two bricked UPs in a row and is waiting for Jawbone to send him another replacement. My UP hasn’t bricked, but the battery no longer stays charged for longer than 3 days at the most (it’s supposed to keep a charge for 10 days). I have not yet contacted Jawbone, but I am going to.
Am I bummed out that my UP doesn’t work just right? Of course. Do I wish I had bought a Fitbit instead? I don’t think so. Once I resolved that the UP didn’t have to be on my wrist 24/7 I began seeing it as a different type of device: one that reminded me to get up and move around every 30 minutes or so, and one that could quietly wake me up at an opportune time in the mornings.