The core applications (Word, Excel etc), other than Outlook, are practically the same. Outlook is the only compelling reason for a desktop user to upgrade IMO. Business users will see a benefit in the collaboration tools offered by the new Office. So long as the business decides to support these features at the backend. All in all, If you have Office XP, I wouldn't upgrade. Unless I did an upgrade of Outlook. If you have an older version of Office, the upgrade to 2003 would be a better choice.
\Dan