I use it to:
1) Sync my own files between portable/desktop. I use my laptop to take notes at volunteer Board meetings, but I like to work on the files on my desktop, and then have them available to the laptop at the next meeting. By setting up the doc folders inside Dropbox, it's seamless - and best of all, mindless. I don't have to "think" about synch the files back and forth, they're just there to be used regardless of where I am - and regardless of whether I'm connected or not.
2) To share common files needed for a committee on I'm on. It was easier to get the members to sign up for Dropbox (and bumped my free space allowance) than a Google Group. Plus, nobody has to change how they work to collaborate on a project. They're just doing what they've always done - saving docs to a folder. A Google Group requires people to remember to use it, to sign in, etc etc. It's not a big learning curve, but it's bigger than using a Dropbox folder. Plus I can share the folder to both my desktop and laptop, and work with the docs anywhere.
3) To send large and/or multiple photo files to people who need to work on them. No need to worry about who has a space limited in-box, managing which files you've attached and which ones you haven't, etc etc It's as easy as copying files to a folder, and then sharing that folder. Or, if you've worked with this person before.... just copying files. You can see at a glance which ones are there. You can see which ones they've worked on. I find it's easier than Send It, because - it's just a folder. And - the sharing between the desktop and laptop works still.
I'm thinking of paying for an upgrade so I can use it as a back up solution too.