I think the whole idea of the importance of "first impressions" is just extremely overrated. If I run through the mental catalogue of all the people I'm friends with, or even colleagues with, I honestly can't remember the first impression they made on me. That's probably because it's 99% likely they just put their hand out, said, "I'm [insert name]," and that was it. It probably came in the context of many other such greetings that day as it came on a first day of work, or school, or whatever it was.
I wasn't really judging them to see how their smile looked, or whether there was a ketchup stain on their shirt, and I can assure you that I wouldn't have cared either way.
In every case -- whether it's friend, colleague, boss, or whatever, the opinion I've formed has been molded by hundreds or even thousands of interactions. The first one was probably about as (in)significant as the 508th.
I do believe, however, that there's that 1% or so of occasions where a first impression can be exceedingly good or bad.
One civilian and one military example come quickly to mind where I met a person who was so instantly rude and/or obnoxious that it basically seared that impression into me (to protect the guilty, I'll withhold names and circumstances here). I think in both cases, however, it wasn't some isolated thing where an otherwise great person stepped out of line and acted out of character. In both cases, the person was acting more or less in line with their usual self -- the first impression really wasn't anomalous.
The same concept applies for great impressions -- they're equally strong, but also equally consistent with character. One person I'll single out here is Jean Ngangi for going out of his way to welcome me into Community Christian Fellowship (the non-denominational church on Princeton and Stevens) when I was coming in as a complete outsider. But again, I come back to the first point -- whether it's over basketball downstairs or at Dunkin Donuts in the Highlands talking about Rwandan economic development, the same warm, welcoming open-mindedness is coming through.
And even after writing all this, I'm still willing to admit that some may give a bad, out-of-character first impression for myriad temporal reasons. But as long as you don't "close the case" on someone too quickly, you'll see their character come through, and the first impression should eventually fade into all the rest.