[whpalmer4]

When your computer is online, Dropbox will sync along any changes made to files under its care, including those made while offline. It stores the files on your disk as well as on the server, so you have a copy of them while offline. As an added benefit, it stores a copy at Dropbox's provider, so you get an offsite backup, and it also allows you pretty easy access to the previous versions of the file! You also can access your files from nearly any computer with a web browser; you don't have to have the Dropbox software installed to use it. The Dropbox client makes it look like part of the filesystem, which you want for OmniFocus, but not if you just want to grab or add a file to your collection from an arbitrary machine.

You do have to exercise a bit of care with deleting items stored in Dropbox lest you delete the only copy and propagate that to all of the clients, but in my opinion, it is easier than making sure your changes on one machine to a file stored in OmniFocus go everywhere. It is also true that as a solution for files you want to access on iPad or iPhone you lose some convenience with Dropbox, as you won't be able to go directly to them as you can on the Mac, and some extra action needs to be taken (flagging the files as Favorites) to have them cached in the Dropbox app for offline access. For a Mac-only setup, using Dropbox to carry the files around is pretty much a no-brainer; with iOS devices involved, there are some complications of comparable magnitude to the issues you face with doing it within OmniFocus. Much of the friction is due to iOS trying to prevent full access to the filesystem.

In any case, I would recommend some testing both online and offline to make sure you understand the workings of your tools to lessen the risk of the unpleasant surprise at an inconvenient time!