I love this: Siri driving Twitter, Facebook, etc (I had written about the desire to do this when Siri first arrived). As I have been splitting my time between iOS and Android, I have been using Siri and Google Voice more and more. Siri is better baked into the platform... and therefore sometimes more useful; but Google is faster and often more accurate.
In both cases, I have used them daily / routinely to dictate emails, messages, etc. It takes getting used to: in part because it's a new way of thinking and in part because you need to learn trigger words and pauses.
The next step is for Siri / Google to become responsive and interactive: deliver messages and make commands more natural / conversational.