USB won't supply a whole lot of current. Possibly limited to 100mA to 500mA depending on your USB host. Should be more than enough to run the micro itself, but if you're running much else it won't likely be enough. Especially if you're running servos or other motors.
You may be able to use a powered USB hub to get some more juice to your project, but that will still likely max out at 500mA.