Recently I had a major issue with the cloud based syncing service - Dropbox. Prior to this, I would sing it's praises to everyone -- now, I'm not so sure.
See the way Dropbox works is that, there is no master copy of your data. All the machines that you link up (in my case 6 of them - both Windows and Macs) are the master copies. What I mean by this is, if you delete a file off one of those machines, it will be removed off all of those machines and the cloud copy that Dropbox has.
Dropbox holds onto these deleted files for 30 days, so if you do notice in that period of time, you can go to the web interface and restore the deleted file.
What happened to me in July was, one of my machine's hard drives failed. It was one of the Macs I don't use for heavy work, so as it turned out, I had been a little laxed with how I was backing it up. As I said, it didn't have any important data on it, Dropbox was taking care of that.
So, it turned out, I had a backup of the drive from November last year. I thought okay, well I guess I haven't lost much, Dropbox will just pull down the files I was missing and I'd need to run the usual updates like Java, Flash and OS updates. When I started the machine, Dropbox had been reset and was at the welcome screen, wanting me to link it up. So, I did that and it proceeded to do some syncing.
It was about 5 minutes later that my heart stopped. I noticed on my other machines, in their Dropbox, folders started to disappear. Dropbox was performing syncs, saying, well you don't have these files on this machine, so obviously you don't want them on the other machines. I quickly stopped the sync and reported the issue to Dropbox support.
They advised me that I could restore the deleted files as it was less than 30 days and I then spent a good hour or so, having to drill into sub-directories to restore them. It was a long painful process and it looked like everything was back - or so I thought. I had a shared folder, that I wasn't the owner of, I had edit permission. A friend of mine was the owner and we'd been spending about 2-3 years working on an iOS project. He contacted me yesterday and asked me to make a new build. I went to the shared folder and it was empty. It was also empty on his machine too (and my other 5 machines).
Panic started to set in, 2-3 years of work gone, just like that. Thankfully, I had been doing regular Time Machine backups of my MacBook Air. On this, I found that the files were there as of July 2014 and I restored the 8,000 odd files (~200mb).
So, thank god for Time Machine and I'm now very wary of Dropbox. I've setup a Backblaze cloud backup of my machine, which includes the Dropbox folder, that way I'll have another cloud based backup, if this should happen again.