Since the inception of .Mac (or MobileMe) it has been nothing but a big stinking headache for me trying to get things to sync. I could reset it, but it would only work on the first sync, every time thereafter it would be an epic fail. On top of that if I tried to set my sync interval to automatic, system preferences would just hang.
There are a couple of articles out there such as ars technicas that do a pretty good job of wiping your sync data to a fresh start. What ended up working for me? Repairing permissions. Now things do sync, but I still can’t set my sync interval to ‘automatically’. An hourly setting does seem to work though. Hopefully the next OS X update will help alleviate some of these sync problems that have seemed to plague Apple for at least the past 5 years. Believe me, I love .mac sync but the second someone comes up with a viable replacement for syncing – I’m gone.