In the course of testing, two general facts emerged that are well worth noting. First, an external keyboard is an essential accessory if you plan on doing a significant amount of document editing on the iPad. It doesn't really matter which keyboard you use. I've spent a fair amount of time on both the Apple Wireless Keyboard and the iPad Keyboard Dock, and both are excellent. Less-expensive third-party keyboards mostly work well, too.
Second, Dropbox is a phenomenally valuable app for anyone who uses the iPad for frequent document editing, as it acts as a file manager on the device and includes integrated links to all of the apps in this roundup. This means that you can use Dropbox to browse for a document in your synced folders and then launch it in Pages, Documents To Go, Quickoffice, Office2 HD, or just about any other compatible app.
Byte Squared Office2 HD
Within the realm of all-in-one office apps for the iPad, Office2 HD has been a leader from the beginning. It's not nearly as robust or intuitive as Apple's iWork apps, but it packs far better cloud support.
Office2 HD creates and edits Word, Excel, and PowerPoint files, and treats Microsoft Office formats as native. You can set the app to use .docx or the older .doc format as its default. It also views iWork documents and PDFs.
Office2 HD's Microsoft Office compatibility is generally very good. Partly because the app itself lacks many of the editing capabilities of its Office counterparts, you'll find it fairly tough to create a document in this app that Word, Excel, or PowerPoint won't preserve pretty much perfectly. And in my tests of importing complex documents containing special formatting, tables, and charts from Word, Excel, and PowerPoint, the app did a good job of displaying formatting options that it couldn't create on its own.