Selling a product that had a voice command UI we found that people will go out of their way to mess with it. Sometimes for fun, sometimes to unload their frustration. There is something strange about a voice UI and I think it is the expectation that the intelligence should be about the same level as human intelligence. If it is not there, and it is making the user look silly by making them repeat something obvious multiple times, they will quickly start hating the product as a whole. It's almost better not have a voice UI unless it is pretty darn good.