It depends on your goals... if you want to get your feet wet with robotics development, a TurtleBot is a good place to start. You can learn the basics of ROS (Robot Operating System), play around with sensing (Kinect) for obstacle avoidance, mapping, and navigation. You can even build some pretty fun applications with a TurtleBot (eg. classic beer delivery).

For an extra couple $hundred, you can buy a decent hobby robot arm (eg. Robotis servos) and doing basic kinematics and manipulation. For the cost of a high-end computer, you can go pretty far these days!

EDIT: Also, I highly recommend reading Probabilistic Robotics. Not strictly necessary for many applications, but required reading if you want to understand most academic robotics topics.