With my new 840, pairing of all sensors was a breeze and all connections are stable. HOWEVER, once the headlight's battery does down you will receive a warning on the 840 display, which takes up at least 2/3 of the display and prevents you from seeing anything but the warning....and it doesn't go away or gives you the possibility to dismiss, at least not without going through different screens. At only one of the 5 or 6 I have, I found a dismiss button.
First of all, I appreciate a batterly low warning...but it could be smaller and disappear after a few seconds or at least show a dismiss button in a prominent position. Being distracted by your Edge while you ride your bike is a bad thing.