I had a chance to play with OneNote 2007, and I can say probably the two biggest benefit i can see right off the bat is its ability to search and retrieve notes.
You can search file titles, text within files, make images searchable, and tag almost anything with a keyword. Also, the way notes are organized in OneNote seems easier than going through folders and folders to find the info you are looking for.
The only downside in my mind is going with with OneNote will be a one way street. It would be painful to revert the changes in my notes back to my old methods 6 months down the road if i decide to.