I use GarageBand with AudioUnits on my iPad all the time. It works pretty well, though they do make the AudioUnits hard to find. Obviously it doesn't have all the features of Logic (which I also use on my Mac Pro), but it's surprisingly good. And, you can take stuff from GB into Logic later if you want to punch it up or do something that's not possible on the iPad.

You know, I just discovered GarageBand on the iPad myself and I have to say I was impressed. Too bad my 4th gen air has lighting as a connector, otherwise I'd pop in a USB c hub and attach my little NI m32 and be rocking! With that, it would be enough to at least draft ideas on the road.

Full-fledged mobile DAW is one thing I'm surprised doesn't really exist. That would make so much money. I know I'm not the only person that gets sudden ideas that need to be expressed audibly before the inspiration is gone, on the go.

Far from full fledged but I just discovered that Cubase has something for iOS and Android. I got really excited until I remembered how hindered I'd be without VSTs, particularly since I mostly score orchestral.

I do believe the day is fast approaching though I was hoping it was going to be with this iteration.