Pros
I've been working at BlackRock for 1.5 years. I've really enjoyed my time with the company. It's non-hierarchical compared to every other firm I have ever worked at and you have access to the most senior people in the firm. My colleagues are very collaborative and good to be around. It's perhaps the most complex and innovative of the largest global asset management companies so you are always learning new things. Given the size I was expecting a more bureaucratic organisation but have been pleasantly surprised - it's very entrepreneurial and if you gave good ideas people want to hear about them. The firm has only been in existence for 25 years so it's free from the stuffiness of older firms where there is an unwillingness to try new things due to an established way of doing things over the years. The firm has been through numerous mergers and aqcuisitions which in itself has created a very open-minded culture overall. It has such a diverse product range across all asset classes and investment styles that there is a product for every change in the market cycle - i.e. not predominatly Fixed Income like PIMCO or ETF/passive like Vanguard.
Cons
It's a global company but the fact that it is US headquartered is still very evident.