"I'm sorry Chris, I'm afraid I can't let you do that." Until I hear those words coming out of my Xbox, (preferably right before it tries to kill me or something) I'll be resistant to believing claims of voice functionality working with games.
Halo: Anniversary tries its damnedest to make voice commands work. It's not a terrible set up, but it does feel more like a novelty then a functional part of the game. The video above shows a couple of the different functions you can use.
Disclaimer: You might want to have your sound on, or else the video won't make sense.