Just apropos to the thread topic, there was recently a New Scientist article talking to Terry Pratchett where he talks about such software:
"Recently Pratchett has become almost as famous for having Alzheimer's disease as he is for selling 65 million books in over 35 languages. He was diagnosed with posterior cortical atrophy (PCA) two years ago - a rare form of the disease which causes the back of the brain to shrink. He has lost his ability to type as a result and had to dictate Unseen Academicals to his assistant. For his latest book he's using a speech-to-text computer program called Dragon Dictate, which learns as it goes. He has dumped his entire collection of novels onto the hard drive and has been training the program not only to recognise his accent but also to learn the words he uses - orc and yennork aren't in most authors' lexicons. "It's astonishing," he says, but he finds it falls down on punctuation."
The full article can be found here, for Pratchett fans:
http://www.newscientist.com/article/...asy-alive.html