Gabe likes LiquidText.
LiquidText is not cheap. It's free to download on the AppStore but $30 for all of the features I've described. If the iPad Pro only allowed me to use LiquidText, I think it would still be worth it. This is the kind of app that justifies the iPad and the expense of the incredibly powerful hardware Apple is developing.
And I'd say he's spot on. I've been playing around with the app to research posts for 1Password and it's pretty amazing. It's exactly what the iPad and Apple Pencil are perfectly suited for, and if you do it enough, it's worth buying an iPad for on its own.
Read Gabe's full review for all the details.