Pros
Decent salaries for the industry. Very good profit sharing and ESOP plans. Opportunities to work in offices around the world. Opportunities to work with top notch clients. Gensler makes pretty good use of technology to allow you to get your job done more efficiently. A number of senior and mid-level people who work at Gensler are very intelligent and have something to teach you regardless of how much you already think you know.
Cons
Gensler is an organization that began life as a design firm and has, over the past few decades made serious steps toward becoming more than just a design firm. Essentially, I've seen Gensler attempt to move from being a design firm that offers consulting serves to a consulting firm that offers design services, which it appears is what the senior leadership has tried to do. The problem with this is that the firm tends to fall back on design every time and abandon any efforts to branch out, thereby lurching the organization back and forth every few years. There is definitely an atmosphere of favoritism within the firm. If your manager likes you, then, as long as you're good at what you do and do quality work, you'll get ahead and do well in your career. If not, it is extremely difficult to progress or sometimes even survive within the organization. I've experienced it both ways and have seen it both ways with many others. Also, Gensler has long been, and remains, something of a cult of personality -- be it among the leadership or among some of the more "talented" design staff.