Munster recently issued his latest report card on Siri saying:
The primary improvement between Siri in iOS 6 and iOS 7 came from its performance in noisier areas, where Siri was able to translate better through a noisy environment (94% vs. 83% correct in Aug iOS 6). Based on our testing, iOS 7 Siri's net ability to answer questions (factors in questions not correctly understood) improved to 79% from 77% in August with iOS 6.
Notably, in Munster's December 2012 tests, 27% of Siri's answers relied on Google; however, that dropped to just 17% in August and just 4% in December 2013. Apple has shifted answers to emphasize Bing, Wikipedia, and WolframAlpha results.
Finally, when comparing Siri to Google Now, both services received a "C+" score. Google Now has a slight lead in commerce and information and Siri performed better for operating system controls.