If it made no difference, then it's doing its job well.

The difference comes from a need...a need for more wattage then previously existed.
You
assumed you needed more power. Upon use and hearing no discernable difference, you verify that you do not need more power on tap. Now unfortunately, you spent $100 that you did not need to spend...right now. However, you may find the amp useful later on when you do eventually run a headphone that does ask for more wattage the the itouch provides. It may just be that the itouch actually outputs a sizable amount of wattage itself and has little need for an additional amp source. You may very well find that other portable devices offer less wattage on their own and will have a higher need for external amperage.
If the NuForce can operate and not change the sound in a noticeable way, it is doing its job well. This means it does not color the sound or introduce any unwanted information(noise). This is good.
My brother owns one. It works well. It does its job on devices and hardware that require it.