Todd Geist has put together a great video showing how imports will let you down when you’re syncing records. Tricky stuff, but the basics are very well explained in this video. (Import failing begins at 11:40)
FM13 demo file from Todd’s transaction demo; you can download this here.
Todd’s article/video also includes a pretty cool demo file for showing how the sync / import works. It’s not fully fleshed out (i.e. it’s a demo) but it’s unlocked and has got a really nice FM13 interface.
Of course GoZync uses transactions: this is why.
Todd’s series on transactions is also highly recommended.