I have read reviews with similar issues and have experienced it somewhat myself. From what I've experienced...when I plug in the device to charge, the watch cycles on and off, beeping every 20 seconds or so and never really charging. I have to keep adjusting the contacts on the charger and turning the watch on/off manually. After messing with it, it finally starts to charge. It can get frustrating.
When it finally is charged at 100%, I'll unplug it and the screen is blank. However, it is still actually ON and when you press the buttons it beeps. You'll think it's off, but if you don't manually de-power the device, the battery will drain.
That's what I have learned from mine. I've had it now for awhile so I don't think Garmin would send me a new one. So I just make due and cycle the power to make sure it's off when I'm done charging it. I'm curious to see if the new one they send you works. If it does, maybe I'll try to press them for a new one myself.