Hey, a professional animation storyboard artist/designer here, long time Cintiq user (about 10 years). I actually dig iPad's technology. With the exception of tiny cintiqs, all of the cintiqs are HD, so drawing on retina felt awesome, I've never been able to draw so precisely before. Also all cintiqs have really (and I mean REALLY) thick glass which is why you have to calibrate, on iPad the pen is exactly on top of the drawing.
Duet was trash for drawing (even with the upgrade), Astropad on the other hand was quite nice. MBP 2016 + iPad 2017 performed with almost zero lag or artifacts at what looked to me like something close to 60fps. Moving canvas and zooming with fingers is unusable in any of these apps so you need to use the hand and zoom tools, but the lack of buttons on the pen mean you either need that big menu bar on the side or use mac's keyboard. I have the 10.5 model, so the biggest issue for me was how tiny it is, you can't really use it for anything serious, even if you hide all the menus with TAB in photoshop (super helpful feature btw). Also there are no edges to rest your palm, like on 13" cintiqs. Maybe 12" ipad is better in that way.
Lastly the fact that there's no native photoshop is crazy (I just don't see why, clicking tiny buttons was easy with a pen and would work especially nice with external keyboard.)
I'd love it if apple released a surface studio-like iMac with similar pencil technology (just add buttons and a bit of a paper feel).
For those who need a portable drawing tablet, while iPad performed much better than I expected, I'd still recommend 13" or 16" Cintiq, if you need to use it regularly.