It's tempting to think about carrying around an iPad, a good enough stylus, and a sketching app that could replace your sketchbook with almost an infinite number of pages. After trying it, although I'm not 100% sure yet, I don't think it'll ever beat a real notebook.
It's the difference between running and running on ice. Or like running on sand at the beach. When you dig that pen into that paper, there's subtlety in the tactile feedback that you don't realize you'll miss until you try to do the same on a tablet. Say you're on page 32. When you press down and the ink starts flowing, you're also feeling those previous 16 pieces of paper underneath, and maybe the cover, ever so slightly. On an iPad, you're hitting glass. There's an element of distance from the tip of the pen to the ink coming out of it. (To be fair, I haven't tried a retina display yet.) That distance is a miniscule loss in control, but it makes all the difference. Then there's the almost inperceptible lag. Even a fraction of a millisecond has the similar effect.
I use sketches to spit out interface ideas. But so much about designing an interface is organizing information, so with that comes a lot of writing. That loss of control, however slight, is glaring when it comes to writing. You can draw boxes and shapes fine, but the writing and arrows everywhere are easier on the real thing.
I use a notebook every day. Not a journal really, but I use it to organize my thoughts on whatever I'm working on. Maybe a plan, maybe a map kind of thing, but writing words is the joy of having a notebook. Even with a 7, 8 inch notebook, you can write a lot on one page. With an iPad, you can write, but it's got to be bigger and it'll always be clumsier to get it down.
There's also the sense of boundaries and confinement. With software, you could theoretically have an unlimited canvas. But it still feels like the boundaries are your screen. A sketchbook feels like it has more room to breathe. It's because you can see and feel the empty pages and the filled pages before it.
But even saying all of that... I love Paper by fiftythree.