For a lumber yard diorama, fsm-barnstead-lumber-ho-scale, I 3d printed several details at Shapeways, including pallets of concrete blocks and some individual concrete blocks.
As you can see, they came out fantastic, all be it, a little expensive, as Shapeways is not cheap.
I tried printing something similar on my Afinia printer, but with it's software limitations, it will not allow one to print walls any thinner than about .020" thick. I was able to do O scale block that look pretty good. As noted in my previous post regarding the M3D printer, it will print smaller wall thicknesses than the Afinia. If you print them very tall or without lots of support, they will fail, which is why the Afinia software ignores walls that thin.
Since a concrete block is not very tall, and with all the bracing built right into the block, I thought maybe it could be printed in HO scale on the M3D. While certainly no where near as fine as the blocks done at Shapeways, to me, they are quite acceptable as scenery items.
This picture is of the raw prints, right off the printer. The O scale blocks were made on the Afinia, and the HO scale ones were printed on the M3D. The HO scale blocks take a little clean up, like a metal casting might, only they are much easier to file and sand since they are ABS plastic.
The M3D will allow me to print some items I was not able to previously.
I also printed some shelves for a piano store on the M3D, and they can be viewed at this link, http://nvrr49.blogspot.com/2015/09/pianos-ho-scale-3d-printed-at.html