Pros
The company was constantly hiring, and they hired well. For the most part the people were smart, talented, engaged, and fun to work with. That was important, because it was a highly collaborative work environment. There were also opportunities for advancement, with a fairly clear path to success. Those opportunities were merit-based. Good, hard work was rewarded.
Kontras
The company didn't seem to value individuals very highly. There was lots of turnover and little value placed on institutional knowledge. GLG hires smart people recruited from top universities, works them to the bone, and then when they leave, replaces them with other smart people from top universities. They don't seem to miss the people who leave or make much effort to retain their workforce. They're clear from the get go that they expect hard work, and that's great because I'm the sort who can't be happy unless I'm constantly busy and challenged. But I felt consistent pressure to work nights, weekends, holidays...it quickly led to burnout. Any time a client needs something you must jump to action. To some degree that's the nature of client management, but there were no healthy boundaries or even acknowledgement by management that life outside of work was important/valued.