Mayors are readily identifiable heads of a city. They can drive decisions and lead in a way that others cannot, often on the strength of their personality. London has had Boris and Ken - but having a well-known and charismatic local as mayor is one thing; whether that person can lead a city, work in partnerships, attract inward investment or draw powers down from Whitehall is quite another.
There will be challenges ahead. Mayors represent only cities themselves and not the wider city regions. This may be a source of tension, as could having a number of mayors operating in close proximity.
Mayors will also have to work out relationships with bodies including transport authorities and Local Enterprise Partnerships.
Not all cities will be as effective as one another. They may compete for inward investment. Cities may have different powers and differing abilities to raise funds. There will be no one way of doing business in cities.
This will result in a more complicated and diverse political map and what is possible in one city may not be possible in others. Whitehall will also have to change. Powerful elected mayors will be able to insist that Whitehall no longer 'knows best'.
There will also be implications for the political parties.
Experience has shown that mayors have a strong independent streak and may not always toe the party line. Centralised, disciplined party systems will be under strain.
Anyone concerned with policy and decision-making needs to understand the impact that mayors will have.