I've used a bunch of sketch/drawing/note taking apps in the past few years and none stick. The ironic thing is I have the worst handwriting on earth and even on paper not a soul, including myself, can read it. Even so, something about sketching out ideas sounds great and I can't seem to leave the thought behind. I first tried out Penultimate and hated it. It's cluttered and does the whole "we want to look like a physical object even though we only exist on a screen" thing, and no one likes that. I also tried Notes Plus which takes the cluttered idea to a whole new level. I could never get in the habit of using them.
Paper is beautiful though. Scrolling through notebooks feels better than if they were really in front of me. Drawing is seamless and little features like "rewind" where I can remove things I've added quickly, without using the destructive eraser is great.
Paper by FiftyThree is free with in app purchases that will add more tools to your palette. If you have the new iPad, I highly recommend trying it out. And I'll be keeping an eye on FiftyThree. They have some potential for sure.