The manual gives ideas about how to configure and work with Outlook views. The add-in comes with those views pre-defined.
The manual uses categories while the add-in copies those categories to a new field called "Actions" and uses those in certain new views. The add-in still uses Outlook's categories field so tasks will sync to your PDA.
The manual contains many handy hints about using Outlook in general - speed keys, etc. and explains how David Allen ties GTD to Outlook. The add-in makes assigning tasks to projects possible and creates views to review those groupings. The add-in is what I would call - email centric - because it really improves your ability to process emails into next actions.
I learned quite a bit from the manual - and appreciate the add-in for the time it saves me in processing and the ability to view tasks in both their next action context and their project context. Both are good.
If you're interested in Outlook, GTD and, perhaps, the next, next thing - you may want to visit http://home.attbi.com/~whkratz/index.htm - Bill's site has another Outlook method, good tips, a forum and he's been hinting at some automation as well.