I don't care about the novelty of Siri's cute answers to questions like, "What's the meaning of life?" And I don't need the phone to talk to me. So I just use Android's built-in Google Voice Search to ask real questions (like "What is the distance to the sun in miles?") or to perform real tasks (like "Navigate to Radio Shack in Sunnyvale" or "Set an alarm for 8 AM," or "Find a Thai restaurant," or "Listen to Rolling Stones", "Call John Smith at home", "Text Jane Doe", "Email Joe Blow," etc.
Some people recommended using "Google Now" but I think that's officially called "Google Search" and is also built-into Android. I have both Google Search and Google Voice Search on my phone, but they're basically the same. The only difference is that Google Search offers a long, thin icon (1 x 4) widget with a space for text entry on the left side and a small microphone button on the right side. If you tap the small microphone button, it activates Google Voice Search. Google Voice Search just puts a bigger (1x1) icon on the desktop. For me, it's easier to hit the bigger Google Voice Search microphone icon on the desktop-- especially when driving a car or something. Since I almost never type a question or command (because talking is so much faster), I keep Google Voice Search on my home screen and I keep the Google Search widget on a side screen.
But I should note that on many Android phones (at least the older ones like mine) there is a physical search button (with a magnifying glass symbol) on the front panel. Short-pressing that button essentially invokes Google Search. And long-pressing that button invokes Google Voice Search. So one could argue that it's foolish to waste screen real-estate on the Google Voice Search and Google Search icons if you have a physical search button. (But I always forget to use the physical button, and for me it's easier to tap the Voice Search icon when the phone is mounted in my car and I'm driving.)
It should be noted that the personal assistant apps that are always on listening for your commands are also always therefore draining more battery.