In a true customer-driven environment, communication is often more important than the traditional on-schedule / on-budget statistics. Indeed, it's quite possible for a project that comes in on-time and on-budget to be perceived as a failure if communication was poor. Likewise, I've seen projects that were late and/or over budget be perceived as completely successful because the customer felt involved. There were no suprises. Same thing with the quality of the product being delivered, which is also often overlooked in metrics.
The bottom line is that, if customer expectations were met and they felt involved throughout the project, they'll be happy.
So, in addition to tracking the traditional completion metrics, be sure to measure the quality of the service and the quality of the deliverables vs. expectations. Your customers will thank you and you'll find out how you measure up in those areas.