Google voice search will make several guesses as to what I said. It will sometimes enter a nonsensical, but similar sounding phrase to what I intended into the text box, but will then figure out that it's first guess is nonsense and return the correct search result.

So just based on observation I would say it works pretty good. On mobile I almost always use voice search, and unless I'm searching for an italian name or something weird it almost always hears me correctly. Even in a noisy pub.