Not very useful
At the moment, if you want to edit documents on your iPhone, there are pretty much only two contenders:
Quickoffice
Docs-to-Go.
It doesn't look as if either have quite thought through how and why people would need on-the-go document editing.
If you can afford an iPhone, you likely have a computer. With a big screen. And a real keyboard too. So are you going to be doing all your word-processing on your iPhone - er, no.
Which means you'll be wanting to do a couple of things: synchronise different versions, and send the documents to people (after all, if you're using your iPhone rather than your main machine, its likely because you are out of the office and in a hurry.
At the moment, Docs-to-Go allows you to synchronise, but not send, and Quickoffice allows you to send but not synchronise.
Both need to try harder.