I installed Yamaha NoteStar on my iPad (just downloaded one song so far - the free one). Quite nicely designed. I can see myself using it more as I progress and pay for a few more songs. They have a nice selection of recent pop songs (Muse, Coldplay...) and this sparked some interest in my son (who sings and plays electric guitar). We might even play something together.

I also got iClassicalScores to browse and download music scores from islmp.org. However, this is not really an app to display sheet music when playing. I print them on paper before practicing.

Finally, I've played with the SampleTank and iGrand Piano demo apps which are not that bad. Nothing to do with sheet music but I could see myself bringing my small m-audio keyrig and my iPad when traveling to train a bit.

I see that Toshibe have brought out a 13 Inch tablet. Do you think this may be a better solution for displaying sheet music?

http://www.engadget.com/2012/06/25/toshiba-excite-13-review/

I read that the Musicpad Pro had a 12.1" backlit display. So I guess the 13" Toshiba display should display scores large enough in Portrait mode.