Allison discusses the challenges and benefits of an Agile Center of Excellence.
Let me first say: I don't love the name "Center of Excellence." This is not about starting a group that has a monopoly on excellence or good ideas with an organization. Just the opposite--this is an entity that helps the organization become more excellent, which includes spotting internal excellence and promoting it.
Regardless of what you call it, an Agile Center of Excellence is meant to be a helpful, consultative group. Not a strict instrument of governance or compliance. While the group may help define mechanisms to promote transparency about product and team health, there is real danger in a COE becoming the internal compliance police.
Digging in further to the idea that an Agile Center of Excellence is a helpful, consultative group that helps an organization become more excellent, the vision of this entity is important. I've found it helpful to use an elevator statement format and Jason Little's strategic change canvas to gain alignment on the group's mission.