Well worth it. I used to use cardboard boxes in the trunk (the boxes that hold bundles of blank white paper...think staples or canon) and they worked well.
The problem was when I didn't need to use them and wanted to put something larger in the trunk. They got in the way b/c they didn't collapse and the item wouldn't fit in the box.
These can collapse flat and stack on top of each other. So you can move them off to the side and free up the trunk space. Or even collapse them and put them on the seat temporarily while you use the trunk space.