This is perhaps one of the most interesting applications for 3D printing I've seen in some time. Looks like a lot of thought went into the 3D parts. I say that not beign a huge fan of plastic parts in machine tools nor unsupported rails. The thing is this appears to be a very competent light weight machine.
In a few places I'd rather see those 3D printed parts be used as patterns for Aluminum castings but you can't have everything.
Having gone from a Shapeoko2 to building my own machine I must say that I don't think the machine above is going to be as rigid as it should be. Especially when you consider strapping that big router onto it.