Notion is a pretty solid notation editor for the iPad. It supports MusicXML, Guitar Pro, MIDI files, and can export as MIDI, XML file, or PDF. The developers have uploaded some new videos demoing some of it’s features…
NOTION for iPad – Dynamics/Articulations
NOTION iPad – Score Creation
NOTION for iPad – Note Entry
Uploaded by NotionMusicMobile on June 5, 2012
The app is priced at $14.99 which I think is pretty fair for a notation editor of this functionality.