I'm using OneNote mostly for my Project Support materials (and my inbox for quick notes). I use it on my Tablet PC - so am using digital ink and typing notes in.
I don't track my next actions in OneNote, although I do sometimes create Outlook tasks from OneNote project materials (which then sync across to My Life Organised)
I've found OneNote to be a fantastic piece of software that has, until recently, been hidden away and not really marketed. As announcements start appearing about the next version of office, apparently OneNote will be included with all version of Office (which is great news).