Peering of Content Delivery Networks (CDNs) allow providers to rapidly scale-out to meet both flash crowds and anticipated increases in demand. Recent trends foster the need for a utility model for content delivery services to provide transparency, high availability, reduced investment cost, and improved content delivery performance. Analysis of prior work reveals only a modest progress in evaluating the utility for peering CDNs. In this paper, we introduce a utility model and measure the content-serving ability of the peering CDNs system. Our model assists in providing a customer view of the system's health for different traffic types. Our model also captures the traffic activities in the system and helps to reveal the true propensities of participating CDNs to cooperate in peering. Through extensive simulations we unveil many interesting observations on how the utility of the peering CDNs system is varied for different system parameters and provide incentives for their exploitation in the system design.