Getting 6 of these guys for 4 points is awesome. I finally gave them a run yesteray, and their first game out they only managed to kill Lylith. They went from frustatingly obnoxious to use, to elegant and simple. They now effect the game.
The biggest thing is the speed. Moving 6" underground per turn is perfect for getting into position behind enemy lines. They cause all kinds of havoc for the opponent because they start playing games with their direction facing so as to avoid the rear arc shots.
They were very good. I know it is hard to make room for them what with all the Krielstone, Chronicler, Fell Caller, etc costs, but if you do make the room, they will not let you down.
Very nice with Grim Angus's feat, or if Grissel can get Calamity on something.