As an avid Dropbox user, I tried accessing my QuickBooks file on multiple computers in the past, but this caused headaches such as conflicting copies of the same file. This was probably a result of syncing issues. Now, I just keep my file in there to keep it "backed up" into the cloud, but only access it and use it on one computer.
Recently, I discovered Qbox. This is a free (using the Basic plan) service that shares your file like Dropbox, but actually locks the QuickBooks file (Word, Excel, and PowerPoint files too) when in use so that there aren't any conflicted copies. I tried the whole process from installing on my laptop and opening my existing QuickBooks file and it was a very easy and seamless process. Keep in mind that you still can only have one person open the file and work in it at one time. Once finished, the changes are synced back into the cloud while alerting the other user that the file is currently locked and cannot be used to make changes. There is also a great feature where you can just open the file in read-only mode so that another user can at least look at stuff but any changes won't be saved. I have super fast Internet and my QuickBooks file size is pretty small, so the syncing didn't take that long. However, it didn't sync as fast as Dropbox, which has a preference to not limit bandwidth, so I am wondering if Qbox has some sort of limit in place. Other than that, I'm pretty impressed with this solution to a problem that has existed for a very long time.