Lots of experience managing projects and teams. Experience working wit large and small business clients. The biggest benefit is the ability to make your own schedule. Exposure to real world business experiences. Internship experience is better than any other paper pushing internship.
Cons
Ambiguity in projects forces you to rely on your own intuition a lot, not necessarily a bad thing though. Low pay, almost non existent.
You are able to work hands on with real businesses that have real problems. Brief but great training is provided, and support throughout projects is invaluable. Being put in charge of projects and teams really allows you to "own" the work and results. Not many things compare to helping businesses grow and succeed.
Cons
There are few full-time positions for interns upon completion of the internship. Given the business climate and circumstances, they do a great job.