I love Penultimate and use it every day to sketch out UI concepts and UX story boards on any given project when I'm on the toad. I love their custom notebooks you can make. I design my custom in illustration save as PNG and upload to penultimate. :)
Penultimate is the original and easiest-to-use handwriting app for iPad that combines the natural experience of pen and paper with the flexibility and syncing of Evernote. Lose the paper, keep the handwriting, and remember everything.