Using this technique you can make tunnels or sewers verryyy fast.
This is an interesting approach. It really gives good caves.
For sewers (or other pipes) you can also use curves with an extrude object. It gives clean, regular topology that makes good pipes. What is problematic is that, if needed, the pipes would need to be merged manually in edit mode after being converted.
This is quite a cool idea.
I think it can be very usefull to sculpt organic style modells like caves
Nice tip 
You can too apply a displace modifier with a cloud map to make it more irregular.
@Monster - yes, I may use it even to create rocky outcroppings, and small cliffs, etc. It is useful for environment modeling of many kinds.
@Victorbalbio - Good idea! Yes, adding a displace modifier will bring it along nicely.
Cool! 
I used to do that some time back, never considered coupling it with the decimate modifier or the “poly reducer” script… Wish I’d thought of that