My evidence is anecdotal at best, but I have found Siri to be terrible and my "OK, Google" to be wonderful.
But - sometime in 2014, and I can't really place it - but right around June/August, Siri all of a sudden turned a corner, and her dictation ability got markedly better - so much now, that I don't even bother typing into my iPhone if I'm in a place where I can talk to it - dictation is 99% flawless. much better than my typing, and unquestionably faster.
For whatever reason, Apple hasn't been making a big deal of this - perhaps because they don't want to admit how crappy it was before - but it really is a big deal. Siri is, 3 years later, what she should have been in 20111.
Can't wait to see what the next step in this evolution will be...