Google Voice Search really shines when you’re searching for something that needs to be found on the web. Whereas Siri usually requires an extra click-through to instruct Siri to open Safari, the Google app hits the web with every query; and almost always with quick, accurate results.
I find this interesting, except for one very important thing. I rarely use Siri to do searches. In other words, I don't ask questions, I give commands. I use it (her) to send text messages, dictate quick notes, add calendar items, start timers and other such things. So for me, this second part of the paragraph rings very true and makes Google Voice Search pretty useless.
Of course, Google’s version has its limits; it doesn’t have access to the data stored on your iPhone, so you can’t tell it to send a text to someone or search your contacts.