I imagine that it's pretty useful to be able to press a button and say, "Text my wife and tell her I'm stuck in traffic, I won't be home for a half hour."
Can Siri do this? Is it clever enough to know what you only want to send the part after "tell her" and how does it know which of your contacts is your wife?
The problem with using natural language for voice control is that by it's very nature it isn't very precise. If the phone interprets what you have said slightly differently from the way you intended it then becomes a much bigger hassle than doing the task manually in the first place. Especially in a situation like using it while driving. Trying to frantically cancel a phone call to the wrong person, or respond to a confused text is not going to be very easy when driving.
I haven't used Siri and most of the reporting I have seen doesn't really delve into what does work and what doesn't, apart from it having a few issues with strong accents. For instance if Alice said "send a text to Bob saying that i'm on my way" does it send "I'm on my way" or does it send "Alice is on her way". Can you configure it in any way to tell it how you want it to respond to certain commands?
Does it work at all with non first party apps? If I say "Play the latest album by Passion Pits" does it just assume I want to use the iPod app despite all of my music being on Spotify?