I find Siri useful for certain things.

'Set a timer for 20 minutes' is faster for me than using the clock app.

'Navigate to the nearest gas station' is faster.

Setting reminders is usually faster with Siri. Especially geofences ones.

Finding pictures in a location is much easier with Siri.. "Find pictures at Lake Tahoe"

The main problem is that you have to remember what you can do.