Darmyth said:
I still believe that if you get knocked down with 80lbs on ya and you're trying to hold 15 in your hands that you're not going to be jumping up right away from prone or do anything real fancy.
Whether or not you are correct realistically, the rules don't back this up. If the rules backed you up I would feel a bit better about stone armor. I would also feel better if the stone armor shattered every time it was penetrated by an attack or the wearer falls down. As it is, even if you are correct, there's nothing in the rules to represent this, which makes it a houserule, and not reflective on the quality of the material in the book.
I'm not saying that it's a bad idea to make it burdensome to wear stone armor. I'm just saying that the book, unfortunately, doesn't do so.
Compare:
Full Plate: 1,500gp +8 AC +1 Dex -6 check 20 ft./15 ft.* 50 lb.
Stone Armor: 1,750gp +9AC +0 Dex -7 check 20 ft./15 ft.* 80 lb.
Despite the textual description, the only rules making it more slow and bulky than full plate are one point of armor check penalty and 30 lb. of encumberance. If the extra 30 lb. pushes him into medium or heavy encumberance, there is no penalty at all because the restrictions for these heavy armors are greater than encumberance penalties anyway. Therefore, the extra 30 lb. is completely irrelevant unless your character hits his carrying limit. The reduced max dex bonus is offset by the extra point of AC bonus. Therefore, the only thing that effectively makes this more bulky than full plate armor is
one measly point of armor check penalty.
Since you don't need a 12 Dex to get maximum benefit from this armor (thus you can put those stat points somewhere else), it will often be superior to full plate overall -- even when actively adventuring, despite the text description of the armor.
In addition, the only reason it doesn't completely shatter from the first impact is because, according to the text, it is "specially treated." That's a cop out in my book. I could "specially treat" a zucchini and carve a sword from it. This is just a subjective preference though, admittedly.