My Garmin 510 has always been hit or miss with syncing. What I would do is put it on the charger at home while still connected to my iphone and then it would automagically snyc after some period of time sitting there. Sometimes I could force it to update by disconnecting/reconnecting bluetooth. Or, quitting the Garmin Connect app. I was growing to hate this little dance.
For a while, nothing I would do could make it sync so I used the cable. There was a bad firmware update last year that disabled syncing for a while.
I just upgraded to a 530 because of all this plus I wanted some of the new features. The 530 is rock solid and syncs every time whether over wireless or bluetooth.
Part of me wishes that I bought the 830 because of the touchscreen and slightly easier routing. (I was used to the touchscreen on the 510.) Overall though the 530 is really nice.