This is a truly fantastic site, sometimes I wonder about the merits of later and more esoteric additions to the list; however this place fully warrants its inclusion.
The building itself is pretty impressive and the level of decoration throughout is striking, especially the guilt leather wall coverings. The courtyard is also very nice and many of the rooms look out onto it.
The exhibits themselves are world class, notable are; the two oldest printing presses in the world, original works of many important Humanist philosophers (of whom Justus Lipsius once worked in the house), masses of typographical matrices including originals by Garamond (it is almost certainly in your font collection on your computer still), many book decorations by family friend Rubens with their original printing plates and a 36 line Gutenberg Bible! On top of this are the entire accounts of the Officina Plantiniana starting from 1555, which are included on the UNESCO Memory of the World list in their own right. There is so much more but not enough time to list all on display.
The highlight for me was the Geographical room featuring several globes masses of maps and what is regarded as the first printed Atlas.
The thing that makes this such a special site; and a main reason ICOMOS sighted for its inclusion, is that it is all in situ. These were where most of the items displayed were conceived/ created/ printed, all in Antwerp which was a key centre of early printing.
The free audioguide was exceptional; it did a fantastic job of weaving the philosophical and practical aspects of the Renaissance and Counter Reformation together and illustrating their progression and contemporary relevance.
A world class site and should not be missed, it was the site with which I "completed" Belgium and stands out as a highlight for me. A personal favourite WHS.