If you need to grab your notes from your smartphone or tablet, you can try OneNote Mobile. It's not as useful as other unofficial Office apps, but it has a layout that you're used to. If you're tied deeply into that ecosystem, it's probably the app you should use.
In order to access this app, you have to log in with a Windows Live account. Once you do, the app only controls stuff saved to your Live account's SkyDrive. It won't open files sent as e-mail attachments. You can create a new note from the app and it will be saved to your SkyDrive, automatically. OneNote Mobile even supports pictures in your notes. The app has the same Metro-style presentation that every Windows' app and download has sported for the past few years. You'll feel right at home if you've used anything Microsoft in that time.
If you're married to Microsoft, this app is a blessing. It's easy to use and does exactly what it promises to do, with no hassles.