"Hey Siri!" is an incredibly convenient way to activate your iPhone, iPad, and now... Apple Watch.
Update: The Apple Watch is here, which makes "Hey Siri!" even more convenient but can lead to even more collisions. Have your iPhone plugged in and your Apple Watch on, say the magic words, and how do you make sure the one that responds is the one you want?
The problem with "Hey Siri!" is everyone knows those words and every plugged-in iOS 8 device — and now Apple Watch —recognizes them. That leads to inopportune activations — mischievous and accidental alike. Google's "Okay Google Now" tries to learn your voice; if it hears someone else yell the activation phrase, it hopefully ignores it. That's an okay alternative if you're the only one who ever activates your device, you never get a stuffy nose or sore throat, or Kevin Spacey never manages to adequately imitate you. But if you do want your significant other, kids, or other friends or family members to be able to activate your iPhone or iPad, you'll still want magic words to work universally. You might just want different magic words.