Here's a list of some of my use cases; I'm sure I'll discover more with time. I keep my iPad passlocked with a strong pass, so Siri is sometimes really convenient. Also, I tend to keep the iPad next to my bed, because very often when I go to sleep I get all those ideas about what I need to do the next day.

1) "wake me up at ..."

2) "Remind me to...

3) "Do I have any new emails?"

4) "What do I have to do today?"

5) "What is the EST time?" (the starting time of some live broadcasts were given in EST and I'm in London)

6) "Set the timer for..." (when I study, or boil eggs), "Stop/resume the timer"

I actually use Siri at least a couple of times every day.