I have used ViaVoice/DNS/Office and my verdict was
Office...crap unless you speak fluent american
ViaVoice...fiddly setup,but once trained quite good.
Dragon Naturally Speaking...Also fiddly setup,also good once trained.
These speech to text apps often are harder to use than simple typing,as speaking the punctuation is a pain(especially if you don't actually know what a / or - is)