Pros
BlackRock has unprecedented scale for an asset manager. This gives it a lot of great opportunities to invest profits back into the business and grow in ways that other firms will have trouble competing with. Senior management is generally very smart about "big picture" strategic issues. For employees, the benefits of this are that there is a lot of interesting work, chances to move around and seek opportunities within a very big company, and the firm has the ability (and willingness) to pay well to retain talented people.
Cons
BlackRock has destroyed the culture of BGI, which basically hired smart, decent people and then gave them a lot of room to run their businesses. BlackRock is very hierarchical and NY-centric. Everything requires getting permission from senior management in NY. While senior management in NY is decent, the hierarchical culture means that a lot of managers spend time trying to figure out what the most senior managers want, and positioning themselves accordingly, rather than doing the right things. In addition, BlackRock is very loyal to employees. This is a good thing, except that in the context of the merger with BGI it means that loyalty to long-time BlackRock employees has prevented honest talent assessments and limited the career opportunities of SF-based personnel. Accordingly, there is a talent drain in SF, as people with opportunities elsewhere take them, that I expect to continue for a while. BlackRock management knows this. Unfortunately, they are very suspicious of "disloyalty" and the more talent that leaves the more patronizing and self-defeating their behavior to SF employees has become.