I've been using OneNote for two months now. I like it. It's flexible, aesthetically pleasing, and very easy to use. My primary use of it on a day to day basis is as a digital to do list -- I have a long long checklist. When I think of something, it goes on the to do list.
I still need to develop the right system in OneNote. I watched a few OneNote videos and paid attention to how people organized their OneNote. I have two notebooks: work and personal. My personal notebook has a few tabs--relationships, self, and finances. (Influenced by Laura Vanderkam's TED talk.) My work OneNote is much messier so far, since I'm currently focused more on how to use it than how to organize it. I can handle messy for now. I used to have an article tacked to my wall: "Messy is the new neat." It resonated with me since my style is to let a mess grow and then after awhile I figure out how to organize the material. So I won't beat myself up on having a pretty messy OneNote right now. It seems to be a good container for all my "stuff" -- notes, websites, jotting down phone numbers, and keeping my lengthy to do list. It's a really individual thing to figure out how you're going to use it and organize it. As I explore how it's fitting into my daily life, my system is evolving. I'll set up a notebook and see how it "fits" -- do I use it? Do I think about it? Do I turn to it/remember it when a related issue comes up? But I still need to learn how to use it more quickly for it to feel efficient. I have no doubt that will come with continued use.
The mobile app and cross-platform compatibility has been fantastic. I have it on my phone, my home laptop, and my work desktop. Wherever I am I can jot down to dos and notes. I took notes using OneNote at a meeting I recently attended. I felt a little awkward typing into my phone--I wanted a sign that explained I was taking notes about the meeting, not emailing or Facebooking! One thing I might do at future meetings is use a pen on a tablet instead. I have to assume the stigma of using a phone in meetings will disappear soon enough. After the meeting, having my notes already digitized led to my post-meeting report to my colleagues being done more quickly.
It's a work in process. I'm still using OneNote every day mostly for personal to dos and work to dos, as well as random thoughts and ideas. It's a really nice interface for thinking about goals and priorities. I'm confident it will be a prominent tool in my toolbox from here on out. What I'm not sure of yet is how central it will be with respect to project management (which I do with Trello) and budgeting my time (which I do with Outlook). Right now it's just my digital notebook, but even in that limited capacity it has improved my efficiency around goal setting and keeping track of things that are hard to classify.