Let's get this right today, so that we don't have to bother with it for the rest of 2015. We can dispense with "not-a-word" talk and calling things "wrong" because of stylebook preferences.
Recently on Twitter, literary agent Janet Reid (@Janet_Reid) tweeted, "'impactful' is NOT a word. Why isn't my snotty spell-czech saying so??"
I replied, "Yes, it is. It just happens to be a word you dislike."
Her response: "wait, NO! say it ain't so. You're my favorite word wrangler, how can this be???"
Impactful has a conventional spelling. It has a pronunciation. And since you bloody well understand it, it has a meaning. Thus it is a word. This is beyond dispute.
What is up for grabs is whether you want to use it. You may dislike it, and because you are a free-born native speaker of English, you are entitled to dislike it and shun it, purging it from your speech and writing. You are also entitled to your private judgments about people who do use it. If, to you, the use of impactful indicates the presence of a fatuous gasbag, your opinions are your own, and I will not attempt to wrest them from you. (On these two counts, actually, I am in agreement. I don't use impactful myself, except for illustrative purposes, as in this post, and I allow myself a private sneer when other people use it.)
But, as I have said here before, and doubtless will again, you do not get to legislate your personal aesthetic preferences.