Pros
They are efficient and will always have a decent list of interesting albeit conservative clients. People are nice enough. Free coffee.
Cons
Considering the culture and quality they strive for, the pay here is below-average and does not keep pace with the cost of living-- I never had any spare money for much while working here. Gensler rewards seniority over merit, so don't expect a reward for working harder than you have to. They offer raises only after you've proven loyal and reliable, about 4-5 years in. Work/Life balance varies by studio and project leader. There were periods of time early on when I regularly left the office at 5:30, but if you want to be on "interesting" or high-profile projects you can expect 10-15 hour workdays (no additional compensation, obviously). One month I worked 100 hours of unpaid overtime and only received one comp-day in return. Its very easy to get pigeon-holed as a young designer. If you're good at something you shouldn't expect to ever do much else. The mentorship and professional development programs are ineffective, I never saw any young designers really move up or gain any new areas of expertise. Most leave in frustration after a couple years.