Every turn, indeed, for 12 TUs a pop. Don't forget that you can use them to look through ceilings and floors, too, by switching elevations while targeting them.
And yes, combined with the 'throughwall > 0' weapons (sniper rifle, bolter rifle, coil gun; and in MP only, the chain gun-type weapons, too), they're extremely powerful because you can kill enemies through walls, ceilings and floors.