Originally Posted by henrikc
Noise cancelling headphones are best for constant sound, like airplane noises etc. They work by having a microphone that registers the sound, and it filters out the same waves so you'll just hear some white noise.
On video games it would probably have some effect, same goes for voices, but cars honking and music playing outside won't be filtered that much.
To add to this...
If you're using the "old" analog feedback method, the response becomes a waterbed effect. You're pushing down the low end response, but you're going to have an equal and opposite reaction on the higher end. You can spread the energy out to a small amplitude change, but you're still changing the response. This is why most people prefer isolation headphones to noise canceling.
For the newer DSP based systems, I'm not sure what the newest tricks are, so this may be a non-issue.