Is speech a useful input modality for applications where the user has easy access to a full-size keyboard and mouse? This study shows that a well-designed speech interface can be more effective than a standard desktop application's traditional interface. Subjects are able to build a set of three spreadsheet tables 50% faster using a spoken dialog interface, and they report significantly greater enjoyment in using that interface. However, these advantages cannot be achieved by simply bolting a speech recognition system onto an application's existing interface. We found that this latter approach led to an insignificant 4% increase in efficiency and a devastating 64% increase in errors compared to the standard keyboard and mouse interface. In short, speech-based interfaces have the potential to substantially improve our interactions with computers, but they require significant interface redesign to take advantage of the unique properties of speech.