Yes, iOrgel looks neat!
I have a few sound apps too, though I don't use them very often, I am definitely a visual person.
Thicket is a very simple and enjoyable app. It shows lines one the screen with a simple music, and you can interact with your fingers and by moving the iPad.
Aeolian Harp is much richer, but maybe because of this, I never quite got the hang of it. You draw dots or line and they move around and bounce on each other, creating sounds in the process.
SoundPrism is a simple music-making app with a very easy and nice looking interface. There is a free version that you can try before buying, I think. Of the three, I'd say this one has the best potential, but I can't really say I have done much with it.