It's just a great way to organize your notes. I like the set up and the accessability. I love outlining in OneNote, as you can format it how you want w/o the horrible auto-format crap that word does. Word is great for typing papers and memos, but I'd hate taking notes on it.
Go to the microsoft website, and download the free demo, and play with it for a bit. Everyone seems to like different things about it. I couldn't really get a sense of what was different/special about it until I tried it out myself.