I've been using dropbox to allow synchronization of app data for my android apps, and I'm quite happy with the API and the process in general.
But there is one substantial problem with it.
There is no way to say that you'd like to allow a group of people (via a shared password for example) to have write access to a folder (and only that folder).
If that could be done, it would be possible to do something like install an android app and tell your 5 friends that they can sync their data with you, for that app only.
As it currently stands, my android apps can only synchronize with someone who also has write-access to your same dropbox account. Essentially this means you can only sync and share with your own multiple devices, not with other people.
It would be great if dropbox could add a feature that allowed you to designate a folder as writeable by different people, as long as they all had some access key/password.