I have built one yes, and you're spot on! The rigidity kinda sucks, which is honestly also what I expected. If I changed the design very slightly and used aluminium parts, I think it would be a usable design, but it's not good enough for a 3d printed arm.
It looks like a cool design - I'm mainly wondering how it would perform in terms of accuracy and speed. 3d printed or laser cut gears could probably work to some extent, but with such a long reach of such a big arm, even small amounts of slop would probably amount to quite a bit at the end effector. Also, the speed is of course not completely crucial if the rigidity and accuracy is there, but if we want to actually make stuff it seems the motor would need to be huge to drive it at a reasonable speed.
It looks very cool though - I think the best way to test it would be to finish some kind of brute design and try it out.
And yes, the big printing stuff sounds pretty awesome. It is, as you say, however, obviously important to keep in mind both production time, weight and if the rigidity actually ends up being better.
I think the conclusion I came to after trying my design was that metal just seems too good to try to compete with. I was planning on trying a third iteration using off the shelf aluminium extrusions and brackets and such in order to keep the design simple, affordable and much more rigid (I'd think).
I think this kind of thing would be a productive thing to work on no matter what!