The 3rd party solutions are going to keep getting better, but it does introduce overhead on both machines and latency because you're essentially running video streaming over USB. Steam Home Streaming performance would suggest that you can get good video stream quality at about 25-35ms added latency, assuming both machines are up the task of handling the stream and you're wired.
With low latency on Pencil itself this could still be a solid drawing experience. I just wouldn't buy an iPad Pro at launch expecting Cintiq functionality.
Astropad is pretty nice, I occasionally use it on my iPad3. Unless they seriously improved the latency over both wifi and lightning cable it's not as fun as it could be.. It has nasty artefacts/mpg-like compression appearing when things move, which obviously doesn't happen with a cintiq. If it did work seamlessly as a cintiq replacement I'll likely sell my cintiq.
Hopefully the display quality will force Wacom into upgrading their smaller graphics tablets to compete, too.
I bought Astropad tonight in anticipation of iPad Pro (it's $19.99 right now, down from $39.99). Tested it wireless and wired with my 64gb iPad Air. Pretty impressed, but I was just doodling in Photoshop CS6 to familiarize with how it works. I haven't tried to pair my Wacom Intros Creative Stylus (original) or Pencil (by Paper53) yet.
For doodling/sketching, painting, I'm thinking it'll be really nice with iPad Pro. And when I'm not around my MBPr, I can use Procreate.