It also depends upon the elements present in the group. Obviously, riots and the "mob mentality" are undesirable. However there are ways to minimize these behaviors. In James Surowiecki's book, The Wisdom of the Crowds, he identifies 4 elements that need to present in order to have a wise crowd:

[from wikipedia: http://en.wikipedia.org/wiki/The_Wisdom_of_Crowds]

1. Diversity of opinion - Each person should have private information even if it's just an eccentric interpretation of the known facts.

2. Independence - People's opinions aren't determined by the opinions of those around them.

3. Decentralization - People are able to specialize and draw on local knowledge.

4. Aggregation - Some mechanism exists for turning private judgments into a collective decision.