The Moto X, for example, also has a virtual assistant which you can start with your voice, but it has an extra processor built in that reduces the battery drain of the always-on microphone.
My wife has a Moto X, and the virtual assistant works very well. If the device is turned off, the touchless control only responds to her voice, for security. If I say, "OK Google Now", nothing happens. Anyone can use Google Now normally if the device is on and unlocked. Also you can say "What's up?" and it will read recent notifications. The always listening processor for touchless control uses very little battery. The battery life of the Moto X will rival that of any smartphone.
Neither Siri nor Google Now started out top notch, and I don't expect Cortana to either. But give it time. Microsoft's Tell Me has very good voice recognition. I recall using it way back when there was a phone number to call and ask for information. Maybe it's still active: I don't know. At work when we activate Windows or Office by phone, we're very impressed with how fast you can read the numbers and it still understands. Microsoft has the voice recognition down, at least American English; they just need to get the AI in place.