Pros
Job safety, learning opportunities. One has to monumentally screw up to get fired. Also, the firm en large is very skilled and competent There are reasons BLK became the largest asset manager in the US, and the uppermost management really knows what they are doing. Depending on the group, BLK can reward dedication and hard work very well. If you choose the right team and work hard, you will make a good career and learn things you would not learn pretty much anywhere else. In addition to upper management, BLK has a 'skeleton' team of very smart and competent professionals. If you can become one of them, you are set.
Cons
BLK is very team specific. While work experience on a number of teams (Portfolio management, Financial Modeling) is extremely rewarding and pleasant, other teams can be quagmires, filled with incompetents, for the lack of a better description. Keep in mind that the asset manager side and "Solutions" business are run very differently. Solutions is a software company with few of the glamor and opportunities found on the asset manager side. Watch out for your MD. He or she, more than anything else will make or break your career at BLK. While there is a number of very fair, objective and merit-focused MDs, you may run into a bureaucrat/careerist, who will think nothing of using you as a slave and trample your career to make a few extra bucks for him/herself. Granted, this kind of person can be found in any company, but this is probably the biggest danger at BLK, as the upper management does have a clue and leads the company well.