Evaluating your media agency

One of my colleagues will soon be launching a "Wave" for media agencies.  For those of you who may not know, a Forrester Wave is an analysis and evaluation of a particular market. 

A critical step is defining the 80 or so criteria on which the agencies will be evaluated.  These criteria will also be made available to our clients so they are able to customize the output - or rating - based on their needs.

Among many others, we'll ask and examine these agencies' media buying and planning offers and how they execute; their clients (including retention rate); their corporate strategy; their ability to help deliver insights on specific audiences; and of course, how they approach integrated - or cross-channel - media campaigns.

What are the most important criteria for you?  If you're an agency, what do you expect to be judged on?  And if you're an advertiser, what are the make or break characteristics of the partners you work with?