I use Siri regularly, ever since it first appeared in iOS; and now am delighted to have use of it in Sierra.
It took awhile for Siri to become familiar with my voice and speech patterns, but having learned that, it rarely ever makes an error anymore.
I'll use Siri to
- compose messages, emails, notes...;
- add items to existing lists or start new lists;
- set a timer on my iPhone;
- start or quit an app, open or close a file;
Probably a lot more, but yes, I find it very useful now.