Same, I use OneNote for planning, brainstorming, and storing snippets and random important info. I don't have to worry whether I saved changes, unlike when I used Notepad++ and kept losing notes. I also do a lot of pen-and-paper scribbling, especially when working on an issue for a customer.

I can access OneNote via the web too, so I'm not limited to my Windows workstation for access to my notes (I use two Linux based laptops for inventory and other "on the go" tasks). I think it's one of the best applications to ever come out of Redmond.