An interesting article from Computer World.
Among other issues, the article explores some of the reasons why most people don't use SR. The biggest reason is the amount of training -- both of the user and the engine -- to get to 98% recognition accuracy, which is the threshold at which users feel that SR is actually useful.
"Without help on those issues, about three-fourths of the people who attempt to use SR eventually put it aside and go back to keyboarding, Parks said, and even among those with training the rate is about 20%."