I've collected a number of demos of Siri so you can get an idea of how well it works. First up is one from Macworld's Jason Snell. He puts Siri through its paces, showing the range of questions and requests it can understand — and examples of how it doesn't always work.
Stuff.TV also has a good demo. Siri has an answer for the question "How far away is the moon" and has no trouble with requests such as "Send an email to my mom" and "Wake me up tomorrow morning at 7 am." In this one you can also see how the background noise doesn't interfere with Siri's language understanding.