Other cloud storage tools for the iPad
Three other cloud storage tools may warrant your attention.
One is Microsoft's SkyDrive app, mainly because it comes with Office 2013, Windows 8, and Office Web Apps as a default storage location, so it's essential for Windows users. (There's a separate Pro version for Office 365 business users.) Note that few apps support SkyDrive natively, so you'll need to transfer files.
Likewise, although this popular service is supported by many apps natively, it's a good idea to have the Google Drive client app.
If you use multiple cloud storage services, AppSense's free DataNow app can manage them, letting you move files among them easily from one console. It supports Dropbox, SkyDrive, Google Drive, FTP servers, and WebDAV servers.