Hi Nathan. I’m the one here at Inventables who’s spent the most amount of time working on milling my own PCBs. When I’m not here, I build little synthesizers and noiseboxes under the name Unatronics 
I’ve been working on other projects lately and haven’t had the time to document my latest efforts. But I can tell you that it does in fact work, and there are some awesome new software tools to help you do that.
Regarding machine stiffness- it may seem counterintuitive, but it requires less rigidity to mill a PCB than it does for wood. Part of the reason is that you’re only removing a very thin layer of copper from the surface of the board- less than .01". And if you want clean traces, you do that very slowly.
Which also means, if that’s what you’re mostly planning to do, the actual cost of that machine could go much lower- $799 for the basic kit since the smaller motors and etc. will perform just fine for that task.
There is a great write-up by one of our customers Matthew White on his experiences milling PCBs using his Shapeoko 2, which is the previous-generation model (and is less stiff, for those keeping score):
He discusses using the open-source program FlatCAM to generate the gcode from a Gerber file. I have used that myself and it works well. I have also heard good things about the PCB - GCode plugin for EAGLE.
I’m overdue on publishing a new blog post documenting using X-Carve and these new software packages to make a PCB, so please excuse the delay. I’ll try to get that documented and written over the next few weeks so folks have a comprehensive guide to follow.