I've been toying with it at the university and I must admit that it feels better than XP: less irritating messages popping up, faster reactions etc etc.
Besides that, there are some new features:
- some CVS (more people can work at the same document at the same time) functionality
- DRM (determine who can do what with your documents, awful technology IMHO)
- XML documents (MS XML that is!)
- better integration (MS says that)
- new applications (OneNote etc).
From the reviews I read, the same bottomline always popped up: MS Office 2003 can be a very useful upgrade for office use, but the value added for the standard home user is very limited. I'd only switch over if you can need/want the new features OR you can buy it for a very low price (students licence or so :)).
My personal bottomline: yeah yeah yeah I know... everyone uses MS Office, but OpenOffice (www.openoffice.org) wouldn't be a bad choice either... and that one's for free... (just a thought)