Before buying BirdieSync, I tried a couple of free or very cheap synchronization tools. One feature that they all had, which BirdieSync does not, was the ability to Preview Sync before applying it. I miss that feature very much, as it could have prevented some catastrophic problems. There have been a couple of times when, following an upgrade or when trying to sync to a new computer, all my contacts got duplicated on both devices. If I had been able to click "Preview Sync" I could have seen what BirdieSync intended to do, and chosen NOT to sync until I made sure all my settings were correct.
This is a much-improved workflow I would like to suggest for BirdieSync:
1) Connect device
2) Preview Sync
-- (with NEW FEATURE: Reconcile Conflicts as described in this other feature request)
Some people obviously won't care about previewing and will prefer just hitting one button. So you could add a separate button for Preview, or make it optional in Settings.