I’ve been reading about this since the day Apple came out with Siri.
My opinion? Two completely different services. Yes, Siri may have the upper hand when it comes to get simple information (What’s the population of Congo?) or pertinent information relating to a certain situation (Restaurants close to my position, please!), but that’s about it. And the only way this even works is because it is based on other services (Wolfram Alpha and Google Maps).
Apple doesn’t have a search engine, and I doubt they will ever have one. Search engine means Google, and it will mean Google. Just look at Microsoft’s Bing and Yahoo – yeah, some people use them in the US (because apparently any thing by Microsoft is used in the US – I’m looking at you, Xbox 360), but they are an abysmal failure in the rest of the world.
The only thing that Siri means is that people with iPhone 4S’s will stop searching for stuff on Google. And that’s ok, because I have made probably less than 100 searches on Google, having an iPod Touch for over a year.
But computers are a whole different story. I don’t use Google to find simple information. There are other specialized services for that, as I’ve said above (after all, those services are what Siri uses!).
And let’s not forget the MOST important point of all: Siri runs on ONE device. Apple may, eventually, expand it to more Apple products, but that’s about it. Siri will always be Apple only. And Apple will always be a small fraction of the market of personal computers.
So yeah, on the mobile platform, the quick, instant searches for a restaurant, Siri may take the lead. But for any serious stuff, Google will be Google for the foreseeable future.