Google Now for the iPhone and iPad was released Monday. Google updated its Google Search app for iOS adding support for Google Now which as been a big selling point for Android. You can argue Google Now steps into Apple's Siri turf, but the two "assistant" services are quite different. Rather than competition from Google Now, I'd say Siri has company.
Google Now learns user habits and preferences via Google services to predict what information to provide. So before you even ask for it, Google Now displays "cards" on the weather, traffic, and sports scores.
On my Android tablet, Google Now is one my favorites features, but despite some shortcomings, Siri on the iPhone remains the best overall voice-aided assistant app I've encountered on any device.
You'd think a Siri & Google Now duo would be a potent tandem. They are, but the lack of iOS notifications forces you to open the Google Now app to view the information. I'll be trying Google Now on my iPad and iPhone, but I suspect that Google Now will fade into the background on iOS. What Apple needs is a Siri Now.