We have used the drill motors for drive train for several robots now. These motors have some very strict limitations and production differences (like speed variance from motor to motor and forward to reverse) but if consideration is give to correct gearing, speed and torque can be optimized very well. The same can be said for the the Fisher Price motors which we have also used this year as boost motors for four wheel drive when needed. Please note that motors should not be expected to work for heavy loads at low RPM. Extended use in this way WILL cause overheating and eventual failure.
Check the data sheets, they are your friend.
Al