A workmate is also a musician, plays guitar, has various MIDI bits, and I sent him a boingboing review of a MOOG synth app, and a classic sequencer, both of which he bought, about a fiver each. Both are identical in every way to the originals in function, controls and sound, and he literally spends hours using it, three of his mates bought the apps when they saw them, and several others promptly bought or started to save up for iPads, just for music creation.
Orbital have a bunch of iPads set up on frames for live performances now, as well.
If you like sketching, Paper is the app to get, very simple to use, but very good results. I have seen a painting done using Brushes on an iPad, printed off in strips and mounted, it was approx. 3×4 metres in size, and in another room 52 iPad paintings of Yorkshire through the seasons, each one about 2.5′ x 3′. You may have heard of David Hockney…
http://www.bbc.co.uk/news/technology-11666162