practical concideration for op-amp differential amplifier
1) Layout is critical, decoupling caps are required, watch the common mode input range on the amplifier, though if you are using +/- supplies you will likely be OK.

2) Since you are measuring across a low impedance shunt (presumably) the input impedance of the amplifier will be much larger than the shunt impedance so it should have little effect.

3) As long as the input resistors are >> than the shunt impedance you don't have an issue.  

As Null mentioned there are dedicated current shunt amplifier that perform this function.