Edinburgh has the best evaluated festivals: yesterday saw the publication of a major impact assessment report on Edinburgh's twelve main festivals (those who make up Festivals Edinburgh). Following the small 'via' link above this text takes you to the Festivals Ed media release; there's then a link from which you can download the full 100+ page report.
The media release will give you the key findings, so I shall restrict my comments to applauding the thoroughness of their approach in carrying out this research, being open in their methodology and seeking at every stage to reflect the breadth of each festival's impacts. It's a report of its time and an important foundation for future work. It also recognises that there's a policy angle to this, and you could say it hits the right buttons for continued investment in these events.
(I suspect this report will shortly be coming to a Festival and Event Management course near me.)