Nothing useful can be said about the effectiveness of a security mechanism except in the context of a specific application and environment. -- Robert H. Courtney
Perfect security has infinite cost.
Security people often reject novel security mechanisms "because they know how to break them." That is to say, they are not effective. On the other hand, they may continue to rely heavily on other mechanisms, like passwords, that they also know how to break. Most of this is simply habit. It does not really have anything to do with effectiveness.
Effectiveness has nothing to do with whether or not something can be broken. Anything and everything created by man can be broken by man; the real issue is the cost. No mechanism provides perfect security. (Indeed, the last thing anyone wants is perfect security; think about it.)
A security mechanism can be said to be effective if the cost of attack is higher than value of success. The issue is not whether or not a mechanism can be broken but how much it costs to break it.
Since we may not know all the failure modes of a mechanism, we never really know the minimum cost of breaking the mechanism. On the other hand, we often know the maximum cost. The maximum cost of breaking an encryption mechanism is never higher than the cost of an exhaustive attack against the key. Similarly, the maximum cost of breaking a password is the cost of a "brute force" attack. Of course, the cost of attack against a well chosen password can be arbitrarily high. Note that the cost of an attack to the attacker is measured in terms of the resources available to him, their reusability, and how he values them. For example, he may value as cheap special knowledge that he already possesses and that is easily re-used, while he values as dear knowledge that he does not have and which would have limited application once obtained.
A security mechanism can be said to be effective if it behaves as expected in the intended application and environment. One of the possible expectations might be that it would resist a certain percentage, e.g. 80%, of attacks. It might be that it would take more than a certain amount of time to break it.
As with most things in security, we need not know the effectiveness of a measure with a high degree of accuracy or precision for the abstraction of effectiveness to be useful to us.