Pros
Work is on live deals. You gain real exposure to the everyday workflow of an analyst. Feedback loop is very good for work product. It helps you grow and gain skills as you learn how to effectively produce high quality outputs. Weekends tend to be less crushed for interns unless there is a huge fire drill. After my internship here, I am highly confident that my skills are good enough to be a first year analyst anywhere.
Cons
No training and it is very difficult to apply for other jobs concurrently. If you're crushed until 12 pm every day, its hard to have time to find meaningful banking jobs and pursue them, or even practice your modeling. Being treated like an analyst and getting a significant share of the work is a double edged sword. This is only true for full-time interns, not summer classes.
No pay.