Let's get this understood up front: USB Disk doesn't allow you to use a USB drive on your iPad. Instead, it uses the metaphor of a physical drive to explain its function: it stores files on your iPad.
You get files onto USB Disk using Dropbox, email attachments, FTP, Finder (and your local network), iDisk and of course, iTunes. Yes, USB Disk is another file storage app for your iPad, but a nicely designed one. Check out a walkthrough of its features in the video below and see if it is right for your needs.