I have only ever used this circuit for a slightly different purpose....
To me it is a constant current source. As the load resistance changes, the current will remain constant, but the voltage across the load will vary. (I am assuming Vs+ remains constant the way I use it.)
Alternatively it could be a current limiter.... say the load is a motor, and the motor stalls, this circuit would automatically prevent excessive current flowing and hopefully prevent motor burn out.
All of the replys in this thread are correct and valid (at least as far as I can tell with my limited understanding). It is just that the circuit can be used in different ways, and viewed in different ways.