The downside of this message is that if the reader doesn't do the hard work required and risk leaving the relationship, it can be used to justify prolonging a disfunctional relationship.
A co-dependent person feels they desperately need their significant other -- even if that other is abusive or emotionally unavailable. Even though the relationship may be unrewarding or even dangerous, the co-depenent will feel that they can't survive without the other.
The book describes the factors that produce these feelings and paints an attractive picture of life without the desperate need for another person.
With a sound grounding in current psychological thinking, it points out that without resolving the issues that result in this feeling of dependency, the co-dependent is doomed to continue repeating the same pattern with every relationship.
The hopeful message is that one can work out one's "stuck" position using the current co-dependent relationship. The book gives techniques that really work for this. The result is more independence, a more solid sense of integrity or self and a better relationship.
My only concern is that co-dependents in a dangerously violent relationship may not take steps to protect their physical safety during this process -- it is seductive to to co-dependent to hear that they CAN find happiness in their relationship and it isn't always going to work.