When it comes to 2-way communication, Siri is quite limited. Before I go further on this allegation, let me make myself clear that I think Siri's natural language parsing is by no means a small achievement.
Seeing it in action
I spent a little time with Siri today and made this video to share:
Overall, Siri is very good at understanding natural English, to the point that you have nearly no need to learn any syntax to operate it. However, there are still some rules you need to be aware of. By and large, your day-to-day plain English, like "what are the Indian restaurants around here", or "find me an Indian restaurant", or any other ways of expressing this idea, can be handled quite comfortably by Siri, most of the time. Sometimes, you need to repeat yourself. Not a big problem.
But when it comes to a series of back-and-forth between you and the device, Siri can get a little stubborn. As you can see in the video, I asked "When is Christmas" and Siri answered perfectly. Impressive. But when I followed up with the request to "set an appointment on that day", Siri failed to associate Christmas with "that day" as a human being would.
In my test, had I say "set up an appointment on Christmas day" instead of "that day", Siri would have understood me perfectly. We human beings work mostly with implicit meanings in conversations; otherwise we would be a verbose bunch. With Siri, that assumption falls apart. To avoid yelling at Siri for being stupid, you need to start practicing forming complete, self-containing sentences today. Yikes! To some, that's more daunting that typing in an appointment.
Having tried Siri, I must say, as long as you make your statement clear, you can say it however you want and Siri can understand you perfectly. Using Nuance technology, the voice to text conversion is mostly flawless too.
Where Scott Forstall tried to sell the idea of Siri "following along the conversation just like a human being does", I think it is mostly not true. To be fair, that type of conversation is NOT the intended use of Siri anyway, as that capability represents true AI that we human has yet to grasp. Looking at Siri's behavior now, it is safe to say that we have a long way to go before Siri can talk to us like Jarvis to Tony Stark.