There is a lot of debate about how to measure the performance of a PMO and why so many are undervalued. Too often the remit of the PMO is unclear, or they are just looked upon as an admin function... There is often no mention of business value being delivered from the project, or of its strategic alignment.
Is a project successful because it have been completed on time and on budget? No, that just shows good estimating.
The real question is what business benefits have been delivered to justify the investment? If a project runs a bit late or goes over budget but achieves more benefits than originally planned, then it is still successful.
The modern PMO must be a portfolio management office not a project management office. It shouldn't be about compliance to a methodology or use of templates. It should be far more strategic and have performance measures that are linked to driving business outcomes that, in turn, are driven by project based change initiatives... then it will be perceived as valuable.