It's better to stay neutral until the moment when you can say for sure that you know this or that person pretty well to say he is a person who deserves (or doesn't deserve) faith, understanding or attention. It's my life motto since I've made such a mistake.
My best friend's name is Nina, we are friends for many ages; but I can never forget our first meeting. It was in summer when our families decided to rest in the south together. When we drove up to their house my attention was attracted by a little tidy girl in a rose dress, who was sitting to squat and playing with her long blond curl. It was Nina. My first impressions about her were far from my opinion about her now. At first she seemed to me so correct, obedient and shy that I didn't like her at all, but when we began to communicate closer I understood I was mistaken. I think it makes sense to say that when we returned home we were loyal friends.
So I know it for sure that appearances are deceptive. That's why now I never make any conclusions about a person when I meet him or her for the first time. It's not my rule to make hasty conclusions, I prefer to get to know more about a person, then to analyze this information and only then to draw my conclusion. It's very easy to form a negative opinion about a person just because you don't like the way he looks, and it becomes like a barrier for further communication between you and this person, though in fact he can be an interesting, well-educated and talented person...