The comparative evaluations cited by Tim should be required reading for anyone who must deal with Microsoft Office apps. The bottom line is that any of the options will work for relatively simple documents of limited length. They all falter to one extent or another when faced with significant complexity and (in my experience) great length.
Word documents appear to be the least problematic. Spreadsheets and PowerPoint presentation documents are considerably more error prone or lacking in features.
For 90% of users, 90% of the time, any of the Office look-alikes will suffice. But be careful if you're trying to use the iPad as a full equivalent of Office apps that require complex formatting or complicated (and rarely used) features.