Evernote is different than other notetaking apps and software because it allows you to sync all your stuff across all your devices, regardless of what operating systems they use. So I can access my notes on my computer, my Nook, or my iPhone. You do need Internet access to view your notes with the free service (only the paid upgrade actually downloads your notes onto your devices), but having it on your phone, too, resolves that problem pretty easily.
Evernote allows me to pretty easily organize all of my research for my novel. You can create different notebooks full of individual notes, and even group notebooks together under a larger notebook in order to easily locate information when you need it. You can also save all kinds of things as notes: checklists, text notes, voice memos, pictures, URLs, or clippings from web pages. The free service caps you at 60MB of information a month, but your data allowance resets every month, and I find it to be plenty. The only other reason I might decide to go with the paid service is if I find that I need access to my notes when I am offline, either with my computer or my Nook -- like I said, I do have Evernote on my phone, but I may find that's not enough!