Try entering different values (starting at 0) into the Serial Monitor and notice at what value the motor starts to actually turn. You will find that the motor starts to 'sing' as you increase the analog output.
Try pinching the drive shaft between your fingers. Don't hold it like that for too long, or you may cook the transistor, but you should find that it is fairly easy to stop the motor. It is spinning fast, but it does not have much torque.
Simon Monk is author of a number of books relating to Open Source Hardware. The following books written by Simon are available from Adafruit: Programming Arduino, 30 Arduino Projects for the Evil Genius and Programming the Raspberry Pi.