Do you want to get work done, or just read docs?
AirSharing HD & Files HD are great—I've used AS since it debuted on the iPhone—but they're document viewers only.
If you need to do real work & to organize it in a nested-doc/folder hierarchy (as in a desktop OS), try Quickoffice Connect Mobile Suite. It's Word- & Excel-compatible and uses a folder structure that displays in columns, just like in Column View in an OS X Finder window. (If you're a Windows user, sorry—I don't know how modern versions of Windows display files. But I'll bet this is plenty similar.)
It's weird to have to keep your office work trapped inside an app, but until Apple opens up the iPad's file structure to the user, everything's sandboxed mercilessly.
Quickoffice is $9.99, for a while, in the App Store. So far it's the only app I've found which allows me to organize my work in the iPad the way I'm used to after years of PC use. Since I'm using the iPad as a laptop replacement, it's something of a relief.