I'm a Conservatorium-trained composer and shoot me - but I like iOS GarageBand in certain situations. I still use Logic Pro for big projects, but because I'm now often bedridden due to injury, GarageBand is an accessible sketch-board to get started on stuff I can later import into Logic if needed. And now with Audio Unit support its going to be even more powerful.
All "instruments" have limitations and for me all of these software tools are just instruments to express creativity. Just recently I saw a composer create some brilliant filmscores using a crappy melodica mouth organ!
Talk about limitations - yet this guy nailed it!
For my small church band I enjoy the challenge of making backing tracks just using iOS Garageband: "given its limitations how can I stretch myself to make this instrument sound exceptional?"
For other jobs I need "other instruments" like Logic Pro or Ableton.
That being said, I can't see myself using the Live Loops much but I haven't really explored it much. Who knows...