A good article on what not to share with your coworkers - 13 Things Not to Tell Coworkers. It can tough spending 40+ hours a week with these people, and not getting personal. From experience - if you are going to get even a teeny bit personal - make sure it's someone you trust. Like, really, truly trust.
There are things I know about other coworkers that could be damaging if they become public knowledge. Luckily, I am not one of those people - that's the part of me that will always be the 'nice' girl.
But still, be careful what you say chickees!