Siri is very well integrated with the 4S. All you have to do is lift the phone to your ear, wait for a beep, then speak. It does a remarkable job of speech recognition (95-98% accurate for me). And it handles some pretty complex instructions.
Some of the interactions I had with Siri yesterday and today:
Me: "When will it rain next?"
Siri: "There's no rain in the forecast for today through this Wednesday"
Me: "Wake me up at seven."
Siri: "You already have an alarm set at 7 am (I turned it on for you)."
Me: "Open the pod bay door."
Siri: "I'm sorry, Michael, I can't do that."
Me: "What is the meaning of life?"
Siri: "All evidence to date suggests it's chocolate."
Me: "Remind me when I leave the house to bring the receipt." (I had to go out and run some errands this morning, and I needed a receipt to pick some stuff up that we ordered.)
Siri: "Here's your reminder for when you leave home: Bring the receipt. Shall I create it?"
Me: "Yes"
Siri: "Okay, I'll remind you."
(When I pulled out of the driveway this morning, the reminder triggered.)
Me: "What are you wearing?"
Siri: "Aluminosilicate glass and stainless steel. Nice, huh?"
Me: "What day of the week is Christmas?"
Siri: "Christmas is on Sunday, December 25, 2011."
Me: "How many days until Christmas?"
Siri: "Okay, here you go: " (displayed a Wolfram Alpha result showing 71 days)
When typing an email or text (or anything else), you can just hit the new microphone icon and say what you want typed.
It's currently fairly limited on what it can do (play music, make calls, set reminders, use the calendar, check weather, check stocks, text, email, search, get directions, etc. -- i.e., it only works with built-in apps, not ones you download) but I suspect that it'll become even more integrated and all-encompassing as time goes on. I would also expect/hope that Apple would open it up to developers of other applications so that it can use them. For instance, I would love for it to feed into TomTom or GasBuddy, add items to my Netflix queue, etc.
