I’ve been using Dropbox for about 18 months, and before that I was a Mesh fan, and in the dim and distant past I spent many, many hours fighting with Plaxo and ActiveSync (see here for a post from 2004 on my frustrations at the time) - so I like to think I know a thing or two about file syncing issues.
I have a separate work Dropbox account, which for the past couple of months has been so far over its (free) limit that I've had to stop using it (there's only so much you can legitimately clear out!) In the meantime, two things have been happening - I've been adding to the local directory, in anticipation of a future upgrade, and other people have been adding content to the remote folders. I had enough faith in Dropbox already to be able to cope with this – I’d get another 3GB of new content coming down, and Dropbox would get a few MB of changes from my local machine. Easy.
Unfortunately, having upgraded, I discovered that in some overzealous spring-cleaning I had removed critical user profile files, and so the Windows user account associated with the work Dropbox account was corrupt, and I couldn't login and start Dropbox. This meant that all of my changes were now stuck in a local folder that couldn’t be synced, hidden amongst 2GB of data that had already been synced to the cloud.
My solution was to create a brand new Windows account and to copy over the entire 2GB archive to this new profile before hitting the sync button - but this meant that instead of adding just my recent changes, I would be syncing the entire 2GB local directory to the new user profile, 99% of which was already on Dropbox, uploaded from a different Dropbox sync. I was convinced this would result in conflict armageddon (and about 48 hours of file transfers – 2GB up, 3GB down, merge, sync, index.)
I left it running over lunch, came back and discovered... nothing. It had just worked.
Dropbox is an amazing piece of software, and if they don't sell up to Microsoft (Windows 8 - Live Mesh replacement), Apple (MobileMe replacement) or Google (Chrome OS file system) for several billion dollars something ain't right.