Accuracy, Coverage, and Speed: What Do They Mean to Users?
  Speech is becoming increasingly popular as an interface modality, especially in hands- and eyes-busy situations where the use of a keyboard or mouse is difficult. However, despite the fact that many have hailed speech as being inherently usable (since everyone already knows how to talk), most users of speech input are left feeling disappointed by the quality of the interaction. Clearly, there is much work to be done on the design of usable spoken interfaces. We believe that there are two major problems in the design of speech interfaces, namely, (a) the people who are currently working on the design of speech interfaces are, for the most part, not interface designers and therefore do not have as much experience with usability issues as we in the CHI community do, and (b) speech, as an interface modality, has vastly different properties than other modalities, and therefore requires different usability measures. 
Frankie James, Manny Rayner, Beth Ann Hockey