Who do you hang with?

Do you remember how our mothers would always tells us, "people will judge you by the people you run around with"? Well, I do, and now that I'm older, I'm beginning to wonder just how true that is. Do we do that? Do must of us, whether we consciencly do it or not, judge people by who they hang around with? Now, I know us Christians have a hard time with the word "judge"...I don't necessarily mean do we "judge" in the scripture since of the word, but do we see people differently by who they hang with? Has there ever been a time when you met someone, and you didn't know them very well, but you thought they were a pretty cool person, and then you found out who their best friend was, and it was someone you thought was a total jerk. Have you ever caught yourself trying to find a "jerk" part of the first guy that you thought was cool?
Another thought along these lines are, do our friends represent a part of who we are? I mean, I think about all the people in my life that I call friends, and I would be proud for people to "judge" me based on them, because, of course, I think all my friends are cool. But if I didn't think they were cool, and were proud of who they were, then why would they be my friend? WOW, this is getting deep.
Anyway, what sparked this is I have a coworker...we'll call her Jill. I don't know Jill very well, but from the conversations I've had with her and stuff, I thought Jill was really cool. Of course I"m a happily married man, so I don't think she is "too" cool, but you know what i mean. Anyway, I found out who her best friend was, and it is another coworker we'll call Jamie. Jamie gets on my nerves, is not a very good employee, and has many other personality flaws (like we all do) that get on my nerves. Well, I caught myself re-evaluating how cool Jill was, strickly based on the fact that her best friend was Jamie. And that bothered me. So I would love other people's input on this subject. Let me know your thoughts!!