OneNote is a great tool!
Cool stuff. I've been using OneNote myself for about 2 years now. When I first came across this amazing tool, I was impressed by how cool it looked. But I couldn't figure out if this software could work for me, so I quickly dismissed it.
A while later, I decided to give ON another chance, taking a more serious look at its features and actually using it to manage small work projects. Very quickly I fell in love with it!
While I don't always use OneNote for each of my projects, I find myself using it to brainstorm ideas for certain work projects and to jot down next actions for each project. I also use it to store all kinds of checklists and information that I can refer to regularly (books to read, travel lists, meeting preparation lists, etc.). For some of my sensitive information, I even have a separate password-protected section in ON.
I have a default notebook in ON called "My Work Projects". Each project has its own page in the notebook (I have about 5 to 8 projects in ON right now). For each project -- say "Launch New Site for Client XYZ" -- I just go to the page and can immediately have access to my NA's and important project support materials in there, such as links, client notes, future ideas for the site, etc.
I like having all my notes for a project in a central location. Of course some of the support material connected with these projects only exist as paper documents, so this stuff goes straight into a project folder stored in my filing cabinet.
I don't use OneNote with Outlook like you do (I don't use Outlook at all). But I find it cool that both programs are a natural fit with each other.