I don’t know if most Mac users would take advantage of Siri, but it could open up some pretty interesting possibilities. Imagine if your iPhone, iPad, and Macs were all front ends to a single instance of Siri that’s optimized for you, learning what tools you use, what information you want, and so on.
I would use Siri on my Mac all the time.
“Set a timer for 20 minutes” would be the most popular, followed closely by “Remind me to do that thing my wife asked me to do.”
Since Macs have no home button to hold down to trigger Siri, I imagine saying “Hey Siri” being the only way that makes sense.