Pros
I've been a consultant at ISA for about a year, coming from another healthcare consulting firm. Due to the boutique nature of the firm, junior level employees are given opportunities that do not exist at larger firms (e.g. ability to work directly with the CEO, regular client interactions, and project management early on). The analyses and strategy that ISA provides for its clients are innovative and exploratory, with new methodologies regularly being developed. ISA provides the environment for junior staff to learn and grow quickly (downside: the hours and workload can be intense), and has committed to investing in its employees through regular training sessions and one-on-one feedback from senior staff. The biggest pro is the culture: the "ISA family" is full of bright, hard working, dedicated people who make the job enjoyable every day.
Cons
Workload can be intense, and hours can be long (but typical of all consulting firms). The abundance of tourists near Herald Square are also quite annoying.