I mostly use software I wrote. I've tried using Astropad with PDF Expert on the mac and this was a painful experience very much like using a pen that was running out of ink. Had to be constantly erasing and having another go at writing something. That experience put me off trying many more apps with Astropad to drive a presentation.
Currently the only reasonably reliable way of putting handwriting wirelessly on a projector with apple, is to run a program on an iPad, use Airserver or Reflector2 on a mac to make a target for airplay, and have the projector hooked up to the mac. This is a fragile setup where the least capable device is doing all the heavy lifting and it only works with apps that can run on an iPad obviously. So you can't show any Mathematica simulations, flash, mac only programs, movies stored on disk etc etc. It's very limited and quirky with the connections.
It would be awesome to have the mac drive everything (which is a solid setup) and have the iPad be an input device on steroids with Astropad. It seems like Astropad is just short of being there already. Just a mode of communication to the mac that assumes the network is poor. If you really want to cut into this space you could add some presenter tools like a pointer mode to draw attention to something on screen for instance.
Really enjoying Astropad by the way!