Is there a significant correlation between PMO's effectiveness and project success in term of time, cost, scope and quality?

There are contradictory views on the values of PMO to the project performance from the feedbacks of management and project managers.

The positive view is that PMO provides a consistent project management processes, tools & techniques to the project managers. However, there is also criticism that PMO added another layer of bureaucracy and could not justify the ROI.

Is PMO providing effective approaches to project managers so that to cut down costs of running projects or is it an entity that incurred additional overheads and operating costs?

Please share your experience.