Microsoft is proud: "Windows Vista includes a built-in speech recognition user interface designed specifically for users who need to control Windows and enter text without using a keyboard or mouse. There is also a state-of-the-art general purpose speech recognition engine. Not only is this an extremely accurate engine, but it's also available in a variety of languages."
But this long video shows how difficult is to browse the Internet if you're brave enough to use only your voice, and not your fingers. To improve the performance of speech recognition, users should perform some training before actually using it. But it's weird to see that only Internet Explorer has problems, while Firefox is responsive.
This reminds me of a Saturday Night Live sketch (transcript), that illustrates how hard speech recognition is.