The new Kinect bundled with the Xbox One has been relatively well received in terms of its effectiveness to pick up voice commands. Indeed, when it isn't working, it could just be that your'e saying something it doesn't understand – it only really knows 30 odd commands after all. Here's a list of all of them:
As Kotaku points out, there's a few odd choices of language in the commands, like the fact that to turn on your Xbox, you say “Xbox On,” but to turn it off, you have to say, “Xbox Turn Off.” However, commenters underneath have pointed out that the best commands are those that skip through menus, letting you directly open something and actually save you some time – whereas a lot of them, simply substitute a quick button press with a controller and aren't actually that useful.
Kitguru Says: Having transport controls via voice command sounds a bit clumsy though. Fast forwarding by voice command? Anyone with an Xbox One let us know if that works well or not?