One of the shops I worked in sold them and they were not unbreakable we had a few pair come back broken. They did get replaced with new skis but I do not think the second pair came with a lifetime guarantee. A friend that had just broke a pair of Fischer Superglass purchased them but complained about them being the heaviest skis he ever skied.
What i remember about Graves skis is that the joke was that the only way that they could guarantee that the ski would last a lifetime was by making the core out of concrete...and that is how they felt when skied.
The other joke was that most skiers would out live the ski company and their guarantee which was no joke because that is what happened.