I've used Dragon, plus some others. I have not found any that were worth the trouble, at least not for actual writing of fiction. The problematic issues for me are these.
1. Formatting. None of the apps I tried were any good at all with the somewhat arcane rules of punctuation and formatting that have to do with dialog. You have to say "paragraph" or "new line". You have to state open quotes, close quotes. Say the comma. Em-dashes can be iffy. In general, I was getting a fairly high error rate, not to mention that saying the punctuation is more disruptive than typing it.
2. Fantasy names and terms. And, in my case, foreign language words. The software will guess at it. The real problem comes when it makes different guesses at different points. If it consistently got a name wrong, then search and replace fixes it. But when there are five different spellings for the same name, then it's a labor for me.
3. Place. When I write an initial draft, I write on paper. This lets me write notes to myself, snatches of dialog, descriptions, speculation, and occasional grousing, all as one more or less continuous stream. No software is smart enough to distinguish, so it all goes into the same bucket and I have to sort it all out later.
There were other occasional bits, but the bottom line was I was spending too much time editing. That had a cascading effect of making me more self-conscious while drafting. I didn't want to be paying attention to punctuation when I was trying to get down a sharp dialog exchange.
On the other hand, it could be useful where such things matter less. Jotting notes. Brainstorming. I've read any number of people who say it's great for writing blog posts. Can't speak to that, but it does indicate it might prove useful for others. I would definitely start with free ones, to see if they fit your work habits.
