Hello from a lurker and bumper... I thought I would provide my two cents on this topic.
I took the plunge and ordered 8 monorail long straights. They arrived today and I have set up a temporary monorail layout by mixing the 4DBrix tracks with the Lego monorail tracks.
From a visual inspection, the 4DBrix monorail track appear to be 3D printed - no surprises there. Consequently, there are very small printing lines visible on the sides and the top of the monorail. Some ends do have small discontinuities but, nothing unreasonable from 3D printing and I have not observed any issues running the monorail.
A visual comment: some AFOLs want as realistic to life MOC as possible, and I will say that the 3D printed monorail track looks more like poured concrete than Lego monorail track.
I was concerned with the 3D printed lines would make a louder noise as the monorail passed over the 4DBrix track compared to the Lego track. The temporary setup was on laminate flooring, which would amplify the noises; however, I could not discern from the noise whether the monorail was on the 4DBrix track or the Lego track (but of course, my wife does say I am hard at hearing sometimes).
In terms of clutch power - the monorail track from 4DBrix is not Lego, but it is on par with what I have experienced (if not better) than the ME Rails I have purchased and mixed with Lego.
In the end, am I happy with my purchase? Yes, I am. I doubled the number of long monorail tracks for half the price of what I could of purchased from Bricklink. More importantly, my daughters are happy.
my two cents.