By my non-scientific survey of family members who use iPhone 4s, Siri is deemed interesting, but it doesn't get used. I find the same thing to be true of Google's "voice search" on my Androids. I think the speech-to-text function works remarkably well. I enjoy having the feature.
But I typically still type in Google search terms. I can't explain why all of us seem to be using text input where speech input is available. In my case I've had the feature for two years, and never have abandoned text entry when searching.
You might argue that Siri is a better, more natural way to "ask questions," rather than "a way to search." But it doesn't seem to have changed behavior much.