ok, so I managed to pick up a 520 at the registration to Ride London from the Wiggle stand.
I've been playing around this morning and setting it up and have a few teething problems.
I have synced with My iPhone 6 via Bluetooth, and have actually found I had to sync with BT_Edge 520 and also BLE_Edge 520. I assume the BLE is for the text / call updates. I can also see that I'm connected on the Garmin Connect app, however I can't seem to transfer Courses between the Garmin Connect app and the unit. I had this capability with my 810, but when I view the course in the app and click "Send to Device" nothing happens.
Also, for the Live Strava segments I have been "starring" a number of segments that will be on my ride tomorrow. It downloaded a handful of these, as I can see than on the Garmin unit, but the rest don't seem to be there.
Apart from these minors issues its all good so far.