Garageband on the ipad is pretty cool Tone - especially for £2.99 - although the ipad costs a wee bit more
You can choose the song key and it selects 8 chords that work in that key
You can generate 8 tracks of music and when you are finished you can then change the key to any major and minor and it automatically changes everything you have played to work with that key
Everything can be transferred into Logic and you can then work on it even more
I love the ipad as I can record and capture an idea anywhere - train or even on the throne
Its a very cool piece of technology - and it even helps me play flats on the keyboard