Google Now vs. Siri vs. Cortana – The Great Knowledge Box Showdown
Having used both of them, I really couldn't live without Siri to be honest.  I'm quadriplegic, and whilst obviously I wouldn't die without Siri my life is immeasurably better with her.Some of the killer features of Siri for me are being able to to write emails, send messages and get quick answers to general questions throughout the day.  She is much better at the first two than she is at the last one though, but even then she is not too bad.And now that Apple has hooked her into much more of iOS in iOS 8, it means that more of the OS is open to me than before just by simply using my voice.  And the "Hey Siri!"  feature is one of the best accessibility features they've added so far.