Unlike the Honda two legged ASIMO robot which publicly fell (twice) while going up (and down) a staircase, the BigDog seems to be a very robust robot. In this video, engineers from Boston Dynamics seem to kick the robot while on the move without toppling it. The BigDog seems also capable of moving on changing terrain including rocks, snow and mud.  

One obvious problem with the existing prototype which can be seen in the video is the BigDog noise level. No combat unit will ever use such a noisy device which might easily compromise their position to the enemy. The reason for at least some of the noise is the one-cylinder gasoline engine which drives the BigDog's hydraulic system. 

Each of the BigDog's aluminum legs has three joints that the computer can reposition 500 times a second using special hydraulic actuators. The joints are fitted with sensors that measure force and position and the on board computer determines where the legs have to be to keep the robot upright and moving in the right direction.

Although the BigDog do not currently have a head itdoes have stereo camera and laser scanner. Future models will also be able to make intelligent decisions about their course without guidance from humans. It is possible that the operational BigDog model will have a kind of wireless communication device which will allow it to track a soldier and follow it around.

Although it will probably be at least 5 more years before the BigDog will see action the Defense Advanced Research Projects Agency (DARPA) is already hard at work on other robotic vehicles such as the 6.5-ton "Crusher" Unmanned Ground Combat Vehicle (UGCV) currently under development at Carnegie Mellon University.