I find my first impression is usually correct. Not all the time and I try not to completely trust my first impression of somebody. But I can usually determine what kind of person someone is after a few minutes. And then the longer I interact with that person the more I can determine if I was correct or not initially. It's not just interacting with a person myself either. I find it's more accurate if I'm observing someone interacting with another person because I don't have to really concentrate on the message but can focus more on the actions and attitude.

It's like if your somewhere with some friends and everybody is having a good time. Then some loud, obnoxious person walks in the room and starts putting everyone on the spot. You know, just being an ass and trying to get attention. Everybody knows what type of person that is based on the first impression and if it's obvious to more than one person than they are probably right. I find I make those assumptions on a smaller scale and they are pretty accurate. But like you said you have to be open minded and the impressions have to be developed and proven over a longer time based on facts.