Just saw this recently. The idea is neat, but the cubes seem kind of clumsy to be useful for any meaningful task.
And they are held together with magnets, which is not so good for power, especially when there are a lot of them, having to connect against gravity or other such force.. A mechanical attachment would be a lot better. http://spectrum.ieee.org/automaton/robotics/robotics-hardware/mit-deploys-swarm-of-selfassembling-robot-cubes