Customer Intelligence - How to measure success?
I'm assuming that the "propensity model" predicts a customer's likelihood to take some desired action.

If it's possible at your organization, then a good way to measure the model's effectiveness is an A/B test.  Select a set of leads to act as a control group using a baseline selection method.  Ideally, the baseline method would be whatever is currently in-place at your organization.  Then allow your model to select a set of leads as the experimental group.  Compare conversion rates (or whatever else you care about) between the control group and the experimental group.

If A/B testing isn't viable, then you can usually evaluate your model using the traditional techniques.  You can split your customer base into train/test sets.  Train your model on the training set, then made propensity predictions for the test set.  Compare the propensity estimates with actual conversion rates.