I have used Dropbox on all my computers minus my DAW for about 3 years now. It is a life saver for people like me who use more than one computer and I'm often away from home. But I also have limited bandwidth at home so have to limit my files to documents and PDF. If I put pictures in it they are web sized.
But I'll add that dropbox slows down your start up as it syncs up so I would not dream of using it on my DAW. It's probably OK if you have lots of power and fast internet but for me it would screw things up. I can still open the files on my DAW using the web but I did not install the app.
For transfers I use my portable hard drive. It's a 2 TB and I make regular full backups of all my Sonar files from the Sonar data drive. I guess my projects are simple because I think my total size of all my working Sonar projects is only 80 gigs right now. I also have finished project folders that I remove from my working drive and back up 6 times elswhere. The working backups are always dated - Sock Monkey-Sept7-2015
My collaboraters live in the same town so I just take the drive.
It would not be hard to save them to a cloud location however.
I save all projects in their own folder with everything assosiated, there will be CWP, a midi file, the audio , lyrics etc.