Well, I just bought the AstroPad app as it is currently on sale for 19.99$. I am such a sucker:D
I have not had much time to test, but forget what I said about soft keys as the on screen Ctrl, Shift, Alt and CMD keys work really well. I was probably thinking of my old Toshiba M200 tablet PC which was not multi touch and where soft keys sucked in ZBrush.
The app is very nicely designed and my first impression of using ZBrush with it is positive. I am running it on an iPad 4 and with the styli I was born with, so I can’t comment on pressure sensitivity.
The only thing that caught me of guard was the way updates to large areas of the screen are displayed on the iPad (such as when rotating a model). The ZBrush interface stays crisp while the model is shown in reduced resolution. I was in a noisy Wifi area and I am sure the Mac that processes the image plays a part as well (I was running on a 2012 MacBook Air) so you might have better results. But it is definitely usable and very snappy.
For that reason I’d probably stick with a bluetooth keyboard and maybe remap a key to the missing stylus button… that difference alone might be awkward for a while, though.
If the AstroPad allowed additional user configurable soft keys that could be positioned arbitrarily I could see myself using those instead. Although there are user defined buttons that trigger a shortcut on the Mac, they are all positioned in a slide out panel that obscures approx. 15% of the iPad screen. On the iPad Pro it would probably not be such a big deal but it would be nice if I just wanted 1 or 2 extra buttons, to be able to place them where I wanted.