Actually, for the moment they should call it Word To Go, as the software allows you to view, edit, and create only Microsoft Word documents. You can view Excel and PowerPoint files, but not edit or create them (yet).
On the plus side, DTG features a companion desktop app (for Windows and Mac) that enables two-way synchronization. And DataViz's InTact technology promises to keep all document formatting intact between your iPhone and your PC.
As for the app itself, it includes such amenities as predictive text, landscape viewing/editing, character formatting (bold, italics, underline, etc.), bulleting and numbering, tables, embedded graphics, and much more.
Most importantly, it allows you to cut, copy, and paste text. (Tell me again why it took Apple three versions of the iPhone OS to add this?)
Documents To Go is currently on sale for just $4.99 -- a "grand opening" price that ends June 30. I'm not sure what it'll cost after that, but you can probably take a hint from competitor Quickword (which is $12.99). Speaking of which, Quickword can't yet edit Word 2007 files, nor does it actually synchronize documents.
In the near future, DataViz will add support for creating and editing Excel spreadsheets, at which point anyone who purchased this version will get a free upgrade.
If your company relies on Exchange, check out Documents To Go with Exchange Attachments, which includes a built-in e-mail client for downloading, viewing, and sending Word files on the run. (In other words, it syncs with your Exchange server.) That version will cost you $9.99 until June 30.
As a longtime Documents To Go fan (I used to use it on my Palm), I'm happy to see it arrive in high style on the iPhone. Now if we could just get a a portable Bluetooth keyboard for word processing on the run. Where have you gone, Think Outside?