I have been using OneNote on Windows desktops and laptops for well over four years. I've used both OneNote 2007 and OneNote 2010. OneNote was the program that convinced me to move from taking notes from simple text files. I had been taking notes that way since around 1989. Generally speaking, I'd rather have access to OneNote than Word, Outlook, or even Access (the database program).
I think that OneNote is the worst-marketted product in Microsoft's stable. People just don't know about it.