Like it or not, dealing with files in Microsoft Office formats is part of business life for pretty much everyone. Looking at Office documents is the less difficult piece of the puzzle; Dropbox offers that functionality.
But for editing and creating Office-format content, you want Documents to Go. Two versions are available.
The standard version (£5.99) allows you to work on Word and Excel files that you create on the iPhone or transfer from your computer by synchronizing with a specific, designated folder.
Moving to Documents to Go Premium (£8.99) adds the ability to create and edit PowerPoint files, and it lets you edit Word, Excel, and PowerPoint files attached to Exchange or Gmail email.
Even if you don't email Office documents as often as I do, this is a case where it's easily worth spending the extra £3 for the Premium application.
Rational people won't spend a lot of time writing lengthy dissertations on the iPhone, but it's nice to know that small changes and edits can be made from anywhere you happen to be.