Pros
As one of the most elite firms in the industry, BlackRock is able to attract and often retain some of the best people in the industry who often really enjoy the challenge of their jobs. In my experience, if you are able to contribute measurably to your group, you will be given lots of responsibility very quickly.
Cons
BlackRock is big and getting bigger, resulting in increasing bureaucracy, though this will depend on the size and hierarchical depth of your group. Having a lot of friends in the tech sector, I sometimes feel stifled by the rigidity of promotion in finance, where promotion is only assessed biannually; differentiation is this regard is only achieved in that the top people are promoted every two years and those who fall behind are simply promoted slower.