If you own an iPad, (in my opinion) the only true music notation product available at this time is Notion for iPad. Notion's goal is to make the iPad version equal to the Windows/Mac version of the product. I've found a few features in Notion for the iPad that might even surpass the Windows/Mac version.
At any rate, the company has been VERY aggressive in updating their app since December, after it released its latest version of the Windows/Mac program in November. I do not know any other app in the App Store that has improved in such as short amount of time as Notion for iPad. At any rate, the company uses Facebook to post updates about the app, and this was their latest update, reflecting a version that will be coming soon:
What is important to note is that a number of users had been complaining on the Notion forum about the inability to write notes with a finger/stylus (entry was limited to an onscreen piano, fretboard, drumpad or MIDI instrument). Notion has quickly addressed those complaints with a new version that will be seen soon.
I am looking forward to all of the changes that are coming…both in the near future and in later versions. Regarding future versions (not the latest version that is coming soon), I am looking forward to swing playback on the iPad (however, you can create swing on the Windows/Mac version and import it to the iPad version and it plays) and also the N-Tempo feature on the iPad, like the Windows/Mac version. It would be great to be able to use the iPad as an accompanist, controlling the tempo directly from the device in rehearsal or performance.
Keep your eye on Notion for iPad (and the Windows/Mac version is a bargain at $99, too). If you don't have Notion for the iPad, it is money well-spent!