Works as advertised. The advantage of this program is that it syncs just about anything that you can sync through .Mac, including contacts, bookmarks, etc. It also has a good user interface.

That said, I prefer address-o-sync for syncing addresses. MySync uses Apple's sync engine. It will inform you if information on cards conflict, e.g., if they have the same work number. However, if information is unique to one card or the other, it assumes that you want to keep it and merges the cards. This can result in bad info. For example, if suppose that the old card for John Doe on machine A has work number 123 and mobile number 456. John Doe has moved, and machine B has a new work number 789 and no longer has his mobile number. When you sync, Apple's engine will inform you of the conflicting work numbers, but will assume, because it's unique to one card, that you want to keep the mobile number, even though it's bad.

On the other hand, address-o-sync gives you three options when any information conflicts on the card: Keep the old card, keep the new card, or merge. It's more flexible, and prevents re-importing bad data. In the example above, I could view the card on both machine A and machine B and reject all of the data from machine A, preventing the bad mobile number from surviving.