Pros
I joined GLG over 10 years ago. It was an exciting journey growing with the company, especially one that continues to lead the market as a global leader. I speak from the client/research operations side of the business that I've experienced, the structure of the organization is relatively flat, "doors" are always open and young professionals are given a fair amount of responsibilities from start. There are a lot of great opportunities for strong performers to move into different roles, the work is dynamic, and there is a lot of room for those who want to do more to take initiative. Appeals to those who are intellectually curious, not shy to just "pick up the phone and ask", and has the aptitude for being in a high-touch client service environment.
Cons
Training/Development for new hires (entry-level or professional) can be a bit haphazard. Overall, I think an individual's experience can depends a lot on the hiring manager and team they join, as well as their own personality fit with the role and willingness to go the extra mile. It's not a job for someone who performs better focusing on a few projects at a time. It's great for someone who thrives on managing multiple tasks (coming from several other professions built on project management skills, this one really demands a strong ability to prioritize and not miss a single detail). You have to like the instantaneous nature of the work (which also means very tight timelines). While relatively open and flat in structure of the organization, the company now has over 15 years under it's belt, 20+ global offices and nearly 1000 employees, if you walk in today and expect a start up type environment, you will be disappointed. There is structure, processes, and a high level of professional expectation that comes with the territory. That said, there's a lot of room for intrapreneurs and those who have proven themselves to take on new roles and responsibilities at a relatively fast pace.