Spent the last couple of days on the phone with Garmin trying to trouble shoot this. The hub pairs and works perfectly with my Garmin 500 so I suspected the 520 to be at fault, but to be sure I also took my Powertap to the local dealer to make sure hub's firmware was up-to-date and otherwise functional (it was).
So this evening Garmin finally concluded that it was likely a defective unit and offered me a free exchange (although the rep would not promise that the 520 was actually compatible with the G3 hub).
The kicker (to the nads it feels like) is that the phone rep said Garmin can't get me a replacement Edge 520 for at least three weeks (!!!) So I'm already kind of annoyed and definitely not feeling keen on waiting nearly another month for what could very well be another $300 box-o-disappointment.
Any words of hope/advice from 520/G3 users--does it pair? I'm close to just kicking the whole 520 idea into the bin and sticking with the proven (and honestly pretty dang awesome) Edge 500
Thanks
