Pros
You will accumulate solid foundational knowledge of finance, economics, the asset management industry and corporate America as a whole. You'll also have the opportunity to interact with teams and individuals across the firm, often on a global basis.
Cons
The company has become very large, making it much harder to work your way up through the ranks. While they claim the promotions process is based on merit, it is also heavily political and focuses much less on individual merit than the firm admits. I've personally found that pay is tied less to the work you do on team and more to what the firm is willing to pay individuals at that level/title, which can be quite frustrating. It is also very difficult to have work/life balance at BlackRock. Although the firm likes to speak of it often and encourage it in meetings, management does not put the right support in place to allow teams/individuals to actually take time off. The firm makes hiring very difficult and most teams are stretched very thin, putting a serious damper on sentiment.