Vantaggi
Pretty dependable that you will work with quality people. I think the relative youth of the company is another big plus, in comparison with IBM/HP/EDS -- sometimes folks complain about new college grads bought in to do important things, but I think it provides a lot of energy and makes the projects more fun in general. I have worked on quite few projects with folks from other companies and absolutely believe Acn is a step ahead when it comes to planning and ability to execute. Obviously not infallible but in my 9 year career I have yet to see a project really fail.
Svantaggi
Obviously the travel is always a major issue in consulting, especially for folks with families or other close ties to home. I generally mitigate this by just moving to the project site, and bringing my wife and daughter along. Daughter hasn't started school yet but this approach won't work pretty soon, I will probably need to start shopping for job without travel requirements. Other than that the hours can be pretty bad if the project is poorly managed, or if you are workaholic, or just unlucky. I have usually not worked weekends or >50 hours a week, but there are the odd situations where folks will work 80 hour weeks and every weekend if the project is in a tight spot.