He is really nice....

you just have to get to know him (or her).

Here is the thing...if I have to put that much effort into getting to know you in order for me to not consider you a jerk - well that is just too much of my time.  If I get to know you gradually because of our working or family connection and I come to see in time that you aren't really an ass then ok - I can accept that and even say "gee, I was wrong about so and so" but in no way am I going above and beyond getting to know you and giving you the benefit of the doubt because someone I know says "he is really nice - you just have to get to know him" - no thanks I would rather spend time with people I like on a natural level.