The answer Jaff, is "it depends!"
Familiarity with people can mean that people will tell you the truth about your performance / behaviour, without trying to get ahead - as Zippy said, constructive criticism. Or you can give them constructive criticism, which they take In a constructive way, because is came from a mate.
On the other hand, being very familiar with some people may mean they take you for granted, do whatever they want, because you're a mate, and you won't pick on them (or so they think)!
I think being in a team, and being familiar with the others in your team is a good thing...as long as people are prepared to give, and to take appropriate criticism, or discipline, should the circumstances arise, even if the person is a good mate.
Pip