I love the Pololu Zumo: It makes it very easy to explore the topic of robotics.
In my case I needed a bit more processing power compared to the Arduino board, plus a few more gizmos and gadgets
The result is a tuned and modded Zumo bot with a powerful Freescale ARM Cortex-M4F running at 120 MHz, capable to run WiFi, USB, GPS and process the wheel encoders in real-time
It uses standard Pololu parts (chassis, blade, reflectance sensor, 1:75 motors with extended shaft, optical encoders), but a custom shield:
Details in http://mcuoneclipse.com/2014/07/26/zumo-robot-with-wifi-and-gps/