What about Christian unity?

In our first gathering together, we think the reality that, while unity is the hallmark of the Christian faith, so often it seems that Christians seem to disagree on numerous points.
What does the Bible say about this? Does disagreement and separation always represent a failure within the church, or are there ever positive examples of healthy, and beneficial partings?
We look at Acts 15 and the different ways the early church responded to differences and disagreement, and specifically about Paul and Barnabas, and their disagreement over John Mark. Is their parting here a failure of Christian love and unity, or could it be that this is a positive example, that teaches us something far more profound about what it means to be part of the Body of Christ?