Pros
I got to work with some very smart engineers and learn new technologies and ways of doing things I wouldn't have been exposed to working in a smaller infrastructure. The company has a lot of employee-friendly policies - high-quality meals, work-from-home once a week, remote workers, etc. The service itself has the potential to greatly benefit society - it gives a voice to a single individual that can be amplified to millions. It almost seems there should be a non-commercial version of Twitter as I feel its that vital. Richard Costolo was also someone that was inspiring and easy to follow.
Kontras
My particular department was run by someone who had joined Twitter during the fail-whale era and he helped stabilize and greatly expand the infrastructure. He was someone that was very easy to talk to and allowed the engineering staff freedom and empowerment to get their job done. However, as the company grew and went public, he decided to hire former friends from past jobs as middle managers. All of them came from older, traditional companies - all interested in their self-advancement, biding their time to continue vesting stock, avoiding disagreement with upper management, saving face at all costs, etc. They created a toxic environment of favoritism, hiring cronies, fear of doing anything innovative or long-lasting, keeping the status quo. It was amazing how little was accomplished for the size, talent and seniority of the team and how long I was there. There were many engineers that were ghost employees - either long-tenured individuals that worked remotely and only for a few hours of a day. Every quarter the teams decided their goals for their quarter. Most of the goals chosen by the middle managers were not necessarily the work that needed to be done but instead the work that could be most easily done. This left gaping holes throughout the infrastructure that only got attention when a crisis would emerge. The only saving grace was there was some very strong engineering leaders who eventually became the de-facto management of the department. They identified the problems and came together with plans to solve them.