I tried out Google Voice Search on my iPod touch 4 (iOS 6) and man, is it fast. Can't do the Siri thing, because Apple won't allow it on my iPt4.
Google Search vs. Siri: Voice search speed test (video)
Google Search (Free) has been on iOS for some time, with well-known search capabilities, access to Google apps, Google Goggles for identifying products, and Voice Search. But an update to the app yesterday enhanced Voice Search with spoken answers, and it quickly became clear Google Search had an edge on Siri -- especially when it comes to speed.