Dan Moren, writing for Macworld:

Part of the reason that the Echo works so well is because it has an array of seven carefully tuned microphones designed to pick up your voice from far away. Most Macs, meanwhile, have limited ambient noise correction on their internal microphones—and most have only one mic, unlike recent iPhones, which use a second mic for noise cancelling. Hearing you say “Hey Siri” may be possible, but it remains to be seen how well your Mac’s internal mic will understand your query from across the room, or when there’s a lot of background noise.

Another potential wrinkle is that we have many devices that can potentially respond to “Hey Siri” these days, including iPhones, iPads, and the Apple Watch. Will Siri know, intelligently, which device to trigger when the phrase is used? Or will we instead be greeted by a chorus of Siri responses from all our various gadgets?

This actually made me laugh. I instantly thought of all my Siris responding to my “Hey Siri”, then arguing with each other about which one I meant to call.

The whole article is worth reading. Some thoughtful insights on Siri’s potential.