One of the best features that I have found in Onenote is the integration that exists with Outlook. It is really great to not have to search for notes or agendas for a particular meeting or event. By linking the two you are able to access information easily from either application.
The other feature that is indispensable in having OneNote and outlook connected is the ability to sent emails to OneNote for storage. Once in OneNote you can then make notes, add tasks to the OneNote, The tasks then are able to help you know exactly what you need to do and capture a better context.