Having worked for an ad agency (and for a consulting firm) AVOID THEM UNLESS YOU ARE A LARGE ORGANIZATION WITH A BIG BUDGET.

They are not going to watch costs for you. They are not going to have their "best people on it" (unless, maybe, you are their biggest client). And everyone working on the project is looking to get their billable hours in. The entire motivation of the organization is to bill as many hours as they think the customer can pay for.

I met a lot of great people in those companies, but I do not miss my time working in them.