I made a pair of iOS apps to help bridge that gap: Composer's Sketchpad[1] and MusicMessages![2]. Above all else, I wanted to make note editing and navigation as simple as possible. At least for the basic ops, there are no tools, no modes, and no switches: what you see is what you get. Unfortunately, this also means that the apps are more suited for rough drafts than anything resembling full, polished pieces. But I've found them remarkably effective for notating quick rhythms, chord progressions, and even solos! They're very simple — there's not even MIDI support yet, only the built-in SoundFont — but I hope to add depth to them over time without compromising on their simplicity.

[1]: http://composerssketchpad.com

[2]: http://musicmessages.io -- will remove from sale to turn into a full iOS app at some point in the near future