When you first launch the application, do not skip the Getting Started screen. It contains useful information for new users. It'll then take you to the main menu, where you will need to select Desktop Files. There, you will register your app with DataViz, who will then follow-up with an e-mail containing a link to downloading the desktop application. This is what will enable the all-important sync between iPhone and computer.
After installing the application, you will need to pair the two as you would using Apple's Remote app. Syncing works well over WiFi and the documents maintain their formatting when going from phone to computer. If the originating document is in another font, that font will be retained when syncing from the computer to the iPhone & back again.
You can either create files on the iPhone itself or use the desktop application to sync with files over the computer. I created my first document on the iPhone. Documents to Go boasts a complete set of text editing tools and pretty much can do anything a basic word processor can do: Font size, appearance and color; applying a highlight to text; justification; creating numbered and bulleted lists; find and replace; word count; zoom in and out; auto-jumping the cursor to different points in the document and undo/redo commands. What it does lack is a built-in spelling/grammar check. Swiping the toolbar will take you to the additional features, and is a nice touch.
The nicest feature thus far? Copy/paste. For real. You can only select all the text rather than just part of it, but copy/paste does work -- within Documents to Go itself. We'll know on Wednesday how well it'll carry to other apps.
Edit (10:15 a.m. ET): Commenter Bobnease points out that if you move the cursor to the word or characters you want to select and hold down, the magnifying glass will appear (something that did happen for me yesterday), but if you keep holding it down, the glass will jiggle and you will be able to drag to select anything that you want to cut/copy. Much appreciation to him for figuring this one out, it was driving me crazy when I couldn't get past the initial magnifying glass!
What's the second nicest feature? If you've got more than one iPhone in the household (which I do), you can set both of them up to sync to different folders on your desktop. Currently, I have mine set up to the default folder and I've created another folder for my friend's documents. Thanks to the ability to sync individual devices with the desktop, you can customize which folders go to which phone.
The biggest drawback is that typing isn't very responsive. When I type out an e-mail message, the iPhone is quick to keep up. But, Documents to Go lags somewhat in its rendering speed, so if you're a fast typist like I am, you find yourself waiting for the application to catch up or missing words altogether. Typing in landscape mode makes the application go even slower.
Another downside, that you can see in the gallery, is that with the keyboard active, there's not a lot of space for the document itself since the toolbar remains active as well. There's even less room in landscape mode. There's also one default font - Times New Roman. While additional fonts would take up precious space on the iPhone, I do wish there was at least a sans-serif font such as Helvetica available.
Despite those quibbles, for $5US, what we're getting is pretty fantastic. There's room for improvement, but we already know there will be upgrades to add Excel editing and more.