I’ve been using Notability on my iPad for over a year now. I find it handy for taking notes on the fly. You can type or handwrite (though it does not do handwriting recognition) I can take pictures of slides at a lecture and caption them on the fly. I also take pictures of any business cards I get handed, which gives me a visual record apart from the little piece of paper that can so easily get lost. You can also record a lecture while taking notes.
You can draw with a finger or a stylus, in a variety of line styles and colors, highlight your notes, annotate your drawings, customize paper color, make it lined, plain or graph, textured or not.
You can store notes by category, and email them to yourself or store them via Dropbox (though not to iCloud), import and export files as .pdfs and .rtfs. What I like best is that it’s really flexible and intuitive. I’m sure there are plenty of tweaks and features that I’ve failed to mention. I leave it as an exercise for you to discover them all.
[Note: If there is a comparable app in the Android ecosystem, do let us know in the comments. --OH]