Siri: Smart, sly, and sarcastic
Siri quickly has emerged as the most talked-about feature on the iPhone 4S. Part personal assistant, part encyclopedia, and part friend, Siri follows your commands and attempts to deliver information when you need it. I spent quality time with Siri during my second day with Sprint's iPhone 4S and asked her a long list of questions. I tried to stump her, and I succeeded at times, but she also surprised me with the range of factoids that she keeps stored away. And like I said, she's got an attitude.
When I first used Siri the day the iPhone 4 was announced, she wasn't able to come up with the capital of Canada. Since she interfaces with Wikipedia, I was surprised not to get an answer, but she did reply correctly when I used another iPhone 4S just over a week later. So, good for her.