Pros
GLG recruits bright individuals from top schools, so you'll enjoy working with your peers. A few of the managers are also very good at their jobs. If you can work with them, you can have a positive experience. The top thing you'll learn from this job is client communication skills. If you're coming from college and have no idea how to write an email or call a client you'll learn a lot of helpful skills.
Cons
On a day to day basis, most people are unhappy with their jobs. Everyone you like will eventually quit. I'll start with the management. While they might be starting to train managers, most are young and have no idea what they are doing. They have no idea how to manage people and very little sense of what is appropriate to say to people below you. People get promoted to manager solely because they have stuck around long enough. Over time I heard a number of manager horror stories, and occasionally heard managers full out yelling at people reporting to them. Senior management is awful at messaging changes and most people feel out of the loop. Efforts at transparency and productivity have morphed into absurd standards of measurement and the number of initiatives on this front is staggering. You'll have to sit through so many useless meetings you will lose your mind. These standards of measurement also mean that you won't be judged on how well you do your job, but how many arbitrary goals you can hit. What used to be a decent culture has descended into a weird "big brother" type scenario. since you can't be trusted to do your job, you'll be watched at every point in time. Working from home is highly discouraged, and some managers even quietly discourage vacation and sick days.