I know many of you will already know how great Garageband is, but for those of you who dont I thought I would put this example up from my daughter, Izzy Marie Hill who has just completed her debut album "The Other Girl" . We used the Ipad2 as a songwriting/composition tool for much of the album, and as an example, here's a "work in progress" video of one of her songs called "Invisible" with Izzy singing along with the Ipad live....
(note that this is a link to a Youtube video of a song work in progress. Its free use so within the Forum rules)
Though we both can play a couple of instruments (piano, guitar), we're neither of us "classically" trained and both first started by basicially picking up the instruments and messing around. However as a songwriting tool the Ipad is great in the way it actually helps you in the composing process - such as the way the "Smart Instruments" process helps you find the best chord progressions based on the key you're in. For non-musical people this may sound complicated, but in reality its an extremely simple and effective process on the iPad2.
So after the "work in progress" stage we took the Ipad 2 into the studio and use the Garageband project to demonstrate to the Producer (and any friends who may be doing a session for us) the form, structure, melodies etc. the song will take.
...and a couple of months later, this is the final track that was produced for the album...
(Note that is a free streaming copy of the song from a free-to-use music website, agan with the terms of forum usage)
Obviously a lot of work goes into the song in the studio, but from the perspective of writing, Garageband is absolutely fantastic at getting the idea down in the first place and then being able to work on it to get it to a studio-ready state