Vantaggi
Bloomberg has its head screwed on internally and treats the development department as a core department, not a cost center. Teams are quite regimented, but there is opportunity for internal transfers. Instead of a christmas party, Bloomberg holds an enormous summer party in the park, families invited, which I found a nice way of handling things. The grad training program is excellent, if extremely hard work.
Svantaggi
A serious case of Not Invented Here, you will often be working with old technologies or internal stacks. At least they are generally of decent quality. The london office felt a bit marginalized by New York. Don't expect to get financial experience - Financial news is not the same as finance, and developers are well isolated from business concerns anyway.