I've had my iPhone 4S for 1 week (as of tomorrow), and haven't used Siri once. I might at some point but there hasn't been a single instance where I felt the need to ask my phone for something. Honestly, I don't know what the purpose of Siri is.
For a lot of internet based tasks, I can open the phone and find out more comprehesive information just as fast (if not faster) as Siri. But...
The Reminders App is GREAT, but manually entering them in, then going to set a alert for the Reminder is such an annoyance and SIRI can do that WAY faster. So I use SIRI for reminders, alarms, and use it to help me in my car through my cars hands free system.
Very rarely. I would rather input calendar events, reminders, etc. myself. The few times that I do use her, she usually interprets my instructions incorrectly.
I use it for texting and spelling. By spelling I mean if I'm doing something and have no idea how to spell a word I just tell Siri and she puts it in text. Then goes on about a web search or some bs.