I installed MtnLion on my SnowLeopard 2009 MacBookPro about a month ago. For the most part it went well with only minor issues and improved performance. This week that all changed as my Contacts suddenly were doubled on the MBpro but still appeared normal on the iPhone/iPad. At the same time I noticed that the MBpro had essentially stopped responding and was showing 99% RAM usage (4GB onboard.)
When i was finally able to get to Activity Monitor to see what was causing the slowdown I saw the iCloudHelper process was consuming 2.75GB of Memory! Assuming that there was some sort of Sync problem I went to iCloud PrefPane to turn off Contacts sync but each time I deselected the box, closed the PrefPane, and reopened it, the checkmark had mysteriously been re-entered. Next I tried to Sign Out of iCloud but that put iCloud Sys. Pref into a hang with the spinning ball. Next I went to Activity Monitor to Force Quit iCloudHelper but the computer did not seem to like that and iCloudHelper popped right back up at about the same massive usage level.
After multiple iterations of turning off syncing/iCloud and turning off WiFi to get disconnected from the iCloud server I was left with my "On My Mac" contacts but the entries were almost completely duplicated. I used Contacts Cleaner to resolve all the duplicates so that the Contacts application looked normal with my ~2200 contacts in 6 or 7 Groups. I then turned WiFi and Syncing back on but soon ran into the same massive iCloudHelper-driven slowdown. That went on for two or there attempts before I disabled sync again and now I had to once again get rid of duplicated Contacts. This time, though, I found that all my Groups had also been duplicated or tripled, but much of the data in the groups was badly corrupted. I figured I could deal with that later.
I then checked the iPhone/Pad and found that all the Contacts on those devices had disappeared, which I then confirmed at iCloud.com. Back to the MacBook Pro to re-enable Sync and iCloud. That restored the Contacts to the iOS units although the Groups were, of course, corrupted.
I have reviewed the other complaints about losing access to Contacts after Lion/M.L. upgrades and see that a variety of solutions have worked to some degree. Apple does not like the idea of trashing the Sync Services folder but does suggest a Terminal reset of SyncServices (http://support.apple.com/kb/TS1627). I have tried that but do not see any help from it.
What sort of success have people had from trashing the Sync folders and forcing a rebuild?
In Snow Leopard one could keep one "Truth" database on one computer and force MobilMe to use that set of Contacts, but that approach does not seem to be available on iCloud/MountainLion. I had really hoped that the sync problems in MobilMe were a thing of the past - but my experience says that is not true.