Cubasis on the iPad is a joy to use, but then Cubase always has had the best user interface out of all the DAWs in my opinion. I think Steinberg have done a great job in what they have done with Cubasis and have shown that it can work on a touch screen.
I think iPADS and the latest tablets are still some way off in terms of raw horsepower for a full blown DAW. Take Auria for example. Great app with loads of features and probably the closest we've seen to a pro spec DAW on the iPAD. However, it really doesn't take much to max it out and thats on an iPAD 3. Auria doesn't even have MIDI yet and if they are to include instruments once they implement MIDI, then it doesn't stand a chance with current hardware spec of the iPAD.
As it stands, I'm more than happy with what Cubasis is giving me. I see it more of a portable song writing tool enabling me to be creative anywhere any time. I've got a more powerful PC for the heavy production stuff. But my priorties maybe different than others. I see myself as musician first, producer second.