Pros
Culture Culture Culture. They work hard to take care of the employees with benefits, make it a fun place to work with events after hours, and keep you inspired with informal design and socializing opportunities. It is the upside of what a big firm can be- they leverage their resources to hire and retain top talent.
Cons
Of course at a large firm, it is easy to get pigeon-holed or stuck in a rut, if you don't take charge of your own fate. There simply isn't the time for leadership to micro-manage enough to ensure that everyone is happy, doing what and headed where they want. That said, they are always supportive if you take the initiative to have those conversations and pursue your passions. You simply need to make your career your own, or it will end up what Gensler needs it to be. But that is no different than any other firm.