I use it all the time for setting reminders and sending texts (especially while I'm riding my bike).
Just a few days ago, I was visiting family in another state. I was flying Southwest, and you can print your boarding pass 24 hours ahead of time to get a better spot on the plane. My mother said she'd set a reminder on her phone so I'd know to do it as soon as I could. Her and my brother (both with androids) were futzing with their phones, trying to set the reminder at the appropriate time, while I just told siri to do it. Took me about 5 seconds to set the reminder, while it took my mother over a minute