I dont use it , but my wife does.
Its very good, but you have to be prepared to put in the time with it, as the more it hears your voice the better it gets
I tried, but couldnt be arsed with the effort.
My wife loves it and uses it well. However one good tip, is that you need to have focus and work with out distractions. If you have to keep answering the phone, emails, etc its harder for it to pic up your style.