To me, first impressions are both important and worthless. Important because there are a lot of things you can pick with one glance: body language, clothing, way of speaking... all of that gives you information about other people, enough for me to have an opinion of them. I can't help it. That being said that opinion isn't fixed; I'm not gonna judge them or be an ass just because guts tell me to. I'm not that much of a jerk, and I do like meeting new people. Whatever I thought before getting to know them has no worth because it's not like I know them.
Unless I'm choosing on purpose to be an asshole, I generally make a good first impression.