I bought The Missing Sync when I got my Treo 680. Synchronization should (in theory) work fine und Mac OS X (Tiger) with iSync and Palm Desktop. However, I had some strange feelings when the Palm wouldn’t sync anymore unless I removed the iSync preference files.
In addition, iSync seems also only able to sync one address and did randomly “forget” some of my contact’s phone numbers or e-mail addresses. The Missing Sync works much better (although I’m not able to sync via Bluetooth right now, I’m trying to get that fixed after I upgrade to v6.0) and it supports multiple gimmicks like videos and music on your Palm device. Definitely worth the price ($39.95 for the full version, $24.95 for the update).