We recently donated a G251X to four students from The University of Southern California for a Senior Project. They made an incredible reactive trajectory system and are using the G251X to communicate with their x axis motor.
The team of 4 decided to test the capabilities of a reactive trajectory tracking system. (Picture below)

"Our theories on contour detection are being tested by building an air hockey robot. As you can see, we mounted a camera above the center of the table. This camera would capture video of the hockey puck moving about the table. The computer would track the puck by detecting it's shape and size (called "contour detection") and predict its trajectory. Based on this trajectory, the system would use a motor and pulley system to move a mallet to the correct position and hit the puck. The motor and pulley is used in a H-bot configuration, similar to those used in a typical 3d printer. The x-axis motor and mallet actually move in unison along the y-axis." -Nicole, one of the teams members.

Click here to see a video of the device in action.
We are very impressed with the work these students put into this project and congratulate them on their success!