It's basically like clay, and it sets very hard. They can't really *tunnel* in it, you're actually meant to pre-mold the tunnels before you use it. It's soft enough that something like a uromastyx or a skink might very slowly carve out a cave, I don't really see a beardie with the attention span or work ethic to use it. Then again, I have a very lazy adult, so you might disagree. Very safe for beardies cuz there's nothing loose to eat by accident, but HARD to clean, cuz it'll absorb yuck and you can't scoop it out. Also, pretty expensive if you're looking to do a whole tank in it. In order to have good tunnels it would need to be quite deep, which means those bags don't go far. It's definitely cool stuff, but I was much more excited by the concept than the actual product.
I get what you're saying about digging though, my beardie dug himself a hole under a little rock ledge to sleep in the first night I got him. Now I keep him on tile (keeps the nails filed!) but he's got a sand box for digging, cuz I just couldn't take that joy away from him.