I have always been guided by my first impressions because they have normally been on the money. In my personal life that is the case - I didn't always follow my own instincts, but in the end was always right. Unfortunately there are times when people just let you down - and that has nothing to do with first impressions. Maybe it just has to do with people not living up to your expectations. Sometimes you need to lower your expectations!
In business I've found that's the case. Not everyone has the same work ethic as you do so it's not fair to judge everyone by the same "yardstick" so to speak (or should I say light meter?)? I think we find after working with people (co-workers, customers) that that yardstick has gotten much shorter for the most part and it's much easier to get along.
Of course there are those FEW that will continue to be assholes - but they are the exception to the rule.