I run all of my business on dropbox. It is baked in to the OS, so you don't have to think about it. Files live on each of your devices plus the cloud (including web access) and includes 30 days (+) of previous versions. Everything syncs well when you go in and out of network coverage. It's easy to selectively share with employees and clients. If they disappear, it doesn't matter since all your files are synced locally to all your devices.

I can't imagine a more secure system. Plus, you get to let someone else manage it for you.