I love Google Now. I sing its praises every chance I get. I haven't been exposed to Siri or Cortana, so I don't have first hand knowledge of how well they work. This is why I try to stay out of the Apple bashing that sometimes goes on.
However, this article from Stone Temple Consulting looks at how helpful these three apps are at providing instantly answers and useful answers. The clear winner: Google Now. And it wasn't even close.
I can't say that I'm surprised, and I'd be lying if I didn't admit to being at least a little smug and superior feeling to my iPhone colleagues.