Pros
Atkins is great at recruiting (though poor at retention), so you are surrounded by highly skilled coworkers with great resumes. Their great resumes mean they can win and you can work on some once in a lifetime projects. The poor at retention aspect means you will have opportunities to step into leadership rolls on projects that you would not get at most other firms. Beware of getting tagged with the clients' dissatisfaction over staff departures though (it's hard to make a client happy when they are on their third or fourth project manager).
Cons
In order to meet the profit margins promised to its stockholders, Atkins is transitioning to a firm that only serves big clients or mega projects. Atkins is dropping its smaller clients by the 100's. If your specialty is big or even "mega" clients, Atkins is the place for you. If not, use opportunities from the high staff turnover to build your resume. But get out before you get the reputation of not being able to deliver finished projects to the client's satisfaction (because your project team members keep leaving). Find yourself a niche at Atkins where you can bill 40 hours a week to one project or client will keep you from suffering the weekly conversations about being more billable.