Just finished building 5 robots. One of them has some problems with the differential and therefore the traction does not work very well. I kept one car so I could use its parts as spare, to overcome problems as the one found in the differential. I also tested the capability of the xbee coordinator to send messages to several end nodes, and that is just fine. There is a small delay that can be noticed (the robots are not perfectly synchronized), but that may be related either to the messages queue or to the hardware differences.
Soon I will post a video of the full construction of a robot.