Are Cults Bad?

Today the word cult conjures up visions of vulnerable people being misled by people with powerful personalities but is that really fair?
Didn't all the organisations of the world start out as small groups of like-minded people who were considered "eccentric" by their peers?
Haven't some cults grown into forces for good?
How do you tell a good cult from a bad one?
What do you do, if anything, about bad ones?