If you want to make quick notes, there is absolutely no better tool than a notebook and pencil.
It's time tested and been proven to work for the past 2,000 years. It offers the minimum possible barrier between your thoughts and the page. It allows completely free expression with no limits on what characters you can type or where you draw your lines. You can erase and rewrite arbitrary areas as many times as you want using an eraser. The battery will never run out, it's very cheap and there's no screen to smash if you drop it.
FYI this isn't the opinion of some ancient luddite who still thinks we should be programming with punch cards. I'm 25 and have been hacking and playing with computer hardware and software since I was 12.
And I still think a moleskin pad and mechanical pencil is hands down the absolute best UI for crystallizing thoughts. If you are using anything flashier you are wasting your time on the tool rather than the substance of what you are thinking.