iSync and .Mac Sync are great. I keep my Treo synchronized with three different Macs, and they all have the same data. Unfortunately, while Apple was enhancing the capabilities of .Mac Sync during the Tiger betas, they broke a bunch of things (including compatibility with Panther). Things have gotten better since then, but I still see problems on about a monthly basis. Symptoms are data not making it over and ".Mac login failed." errors from the .Mac System Preferences pane. Apple has a "fix", but it's pretty draconian, and Sync Services tends to get corrupted again. Here's the procedure:

  1. Disable .Mac syncing (if applicable)
  2. Close all programs. Log out and log back in.
  3. Make sure no programs are running after logging back in. If the user is [the original email I received trails off here. --cp]
  4. Create an archive of ~/Library/Application Support/SyncServices. Delete the directory after archiving it
  5. Re-enable .Mac syncing (if applicable)
  6. Sync with .Mac: go to the advanced tab, click Reset Sync Data and choose to Reset Sync Data on .Mac or on this computer depending on which one has the most recent data
  7. If you have devices, now is the time to manually synchronize them (manually add any new data from the device that is not on the computer), then do a sync with the device choosing Erase data on device then sync

I add step 0: Back up your data (iCal & Address Book have handy "Back Up" items, and Safari has "Export Bookmarks"). When logging back in, hold down the Shift key to avoid automatically launching Login Items.