I tried two document formats, spreadsheets and documents. It's said that presentations are not supported, and I didn't have any, so I didn't test.
The spreadsheet viewer either lets you download as XLS (which I didn't try) to view as HTML. The HTML viewer gives you pages that are 1 column wide and 20 rows deep, so if you are designing reports for mobile viewing you'll want to arrange things in columns, not rows. There's no edit function that I could see. I'll have to rethink the formatting of my Michigan Football Parking spreadsheet and figure out if there's a good way to represent it in this format.
The document viewer successfully loaded a Word file that I had saved from Google Mail earlier and presented it (slowly) for viewing. Some amount of formatting was intact. I tried to edit it, but got confused, and by that time my bus ride was over.
Mobile document viewing and editing has been around at least as long as Socialtext's Miki mobile wiki editor, and I've done successful Mediawiki remote wiki edits on the Ann Arbor civic wiki Arborwiki. It's nice to get a view into Google Docs, but so far I haven't seen more than remote viewing. This is a welcome addition to the Google mobile pantheon, but it doesn't look like it's going to replace my laptop any time soon.