there's heapsssssssssssssssssssssssssss of amazing music apps for the iPad. give beatmaker 2 a try. has lots of great features (ie midi sequencing, loops, samples based songwriting, in-built good quality soundbanks, automation, audiocopy/paste etc) and recently updated to universal. a close competitor to that would be nanostudio that has an amazing synth. it's only made for the iPhone though but you can most certainly run it on the iPad.
i'm waiting impatiently for IK's sampletank for iOS!