One anecdotal data point here. I really, really enjoy not typing whenever possible. Sometimes when I'm driving I have my headphones in so I can treat Siri exactly like a personal assistant. It's useful and only a button-hold away.
If it does change the way we speak then illiteracy rates will likely increase. Take for example words that can be pronounced multiple ways and have two different meanings. I have yet to hear Siri correctly pronounce the word "record" in the context of something that recalls or relates past events.