A member of the LinkedIn PMO Coach forum Ken Martin has comprised a list of 25 possible metrics for PMO performance evaluation http://kigospace.com/public/wp-content/uploads/2013/01/25-PMO-Metrics.pdf
Ken suggests you selective choose from them, I add you choose those which are meaningful to your organisations' business goals. Your PMO needs to fit with what your organization is trying to achieve.
There's some great research out there about PMOs much coming form the Canadian Research Chair at QUAM (University of Quebec in Montreal), Canada - you can read a summary of one of their research projects at https://aupri.athabascau.ca/PHDSummaryMAubry and this site Project Research Institute has another of their research projects under review too (https://aupri.athabascau.ca/node/343)
Research shows that PMOs aren't intended to last forever, they are brought into existence to fulfill a purpose: to support project operations or to control project operations. If an organization doesn't have to have rigourous control (because lives or the safety of the planet depend on it) then a support PMO is usually much more effective but it must be meeting organization needs as well as making lives simpler.
Bottom line: by all means have a PMO but be clear why and how it contributes to your organization's business goals.