I've been using OneNote as my main note-taking app on my Windows-based Tablet PCs. I also have Evernote so that I can sync certain notes across all my mobile devices including my Android smartphone, my iPad, my Android tablet, and my Windows-based PCs.
Microsoft has announced that OneNote Mobile is here. Of course, to sync your notes on your mobile device with your PC, you need to have OneNote 2010 (and I'm still running Microsoft Office 2007 on most of my PCs at home and in the office). For now, I'm not planning on going out and purchasing OneNote 2010 for multiple machines if I can continue to use Evernote.
I plan to continue this ongoing test between OneNote vs. Evernote given that I'm syncing notes across a variety of different platforms include iOS, Android, and Windows 7.
OneNote and Evernote are great pen-based note-taking apps for those who still like to write and draw traditional notes on a digital device. If you don't use a pen, you'll find that these apps are still very handy if you want to sync many different notes across a variety of devices. You can also use a cloud-based app like Google Docs if you don't need pen-based input.