Pros
You will have daily contact with some of the world's smartest investors and industry experts. GLG can very much accelerate your career. GLG's senior management is solid and very competent. Many have experience in management / strategy consulting (e.g., at McKinsey, etc.) or in the financial services industry (e.g., at Goldman Sachs, etc.). The company is trying to plot aggressive growth. Managers are given a good deal of responsibility over their designated book of clients. The company is very good at identifying and promoting outstanding individuals. It is a very meritocratic organization.
Cons
It is an extremely fast-paced environment. You will constantly be on your Blackberry or on the phone with clients / experts. To succeed, you need to respond to client / expert requests right away - with absolutely no delay. Requests will come in 24/7. Typically, you will not do very in-depth analysis in your daily work.