I've had my 520 for a few months and have just bought a Fitbit Aria scale. The Aria sends it's readings from the Fitbit website to MyFitnessPal, which then sends the readings to Garmin Connect. The weight readings show up on the wrong day (the day prior), but I don't use Garmin Connect for tracking weight, so no biggie. Would be nice to have it correct though.
What I've noticed is that when you log a weight reading either manually or via a sync, the value won't be pushed to your device unless you tell Garmin Connect to send it or you go into user settings on Garmin Connect Mobile.
I'd like to see the option to automatically have weight readings sent to any connected devices automatically either by USB or Bluetooth during a sync. Anyone else think this might be a good idea?