Thanks so much for this rundown on tablet painting and for reminding us that tablets are supposed to replace paper. I think they will replace paper in more ways than they currently do, quite soon.
I'm not an artist, but I am curious. How do iPads etc., compare with Wacom's Cintiq or pressure sensitive tablets? Can an iPad or Android tablet, with a 'brush stylus' totally replace them, if the tablet is used as a painting peripheral? If more is needed, what is it? If little separates the two types of tablet for PC/Mac painting, I suspect that the Cintiq's days are numbered.
Who knows what the primary digital painting machine will be in the future - I doubt it will remain what it has been.
iPad or Android is a lot more portable than a Cintiq or Wacom tablet, but you're running tablet apps rather than the full PC/Mac programs - which are more powerful. You don't get true pressure sensitvity on an iPad but you can approximate it nicely by painting lightly with a Nomad Brush or rubbing repeatedly with your finger for smudging effects. What you don't have and won't get without some very expensive hardware is the full range of info that a Wacom tablet/Cintiq can get from the pen - not just pressure and positions but angle (tilt in both the x and y axis) and the ability to sense how that rotates, plus additional information like the setting of a dial on the side of an airbrush. That's ten variables of information that can be used to simulate a complex tool like an airbrush... Software can give you controls to rotate a flat brush sideways but you have to change a setting for each stroke, so the interface for that is hard. Plus full natural media simulation is hard work for even a Core i5 (some oil paint simulations were written to test high-performance computing platforms!)
That said, you can achieve fantastic results on the iPad and other very portable devices. I suspect we'll keep a range of deskbound and portable devices but it's really exciting how things are developing!