Trusting your gut seems to be very different than formulating a first impression. A gut feeling is more visceral and emotionally driven while first impressions are more about the other person's behaviors that gives you cues for your to compare intellectually.

I always trust my gut reaction, but I have changed my initial impressions of someone...gave them a second chance to act differently. I have been pleasantly surprised as well as gravely disappointed...but I wouldn't want to miss out on a potential new friend...two chances are fair in my mind.