Pros
Non work related: the pay is pretty good, the building is fancy, they have free drinks + snacks. There are a lot of outside of work activities that are not only offered but also strongly encouraged, such as donations, food/backpack drives, gardening, etc. Also, there are lots of free tickets given out to cool events in the city that you would otherwise never have access to. Work related: you get to work on projects that affect a lot of people, and depending which group you land into you may love the work you do. The management is less "flat" than what I was told it was like when I joined (early 2009), but there are definitely fewer hierarchical barriers than at most other places.
Cons
Bloomberg has tons and tons of legacy code, mostly in Fortran and C, and to add insult to injury, it's rarely commented/documented/explained in any shape or form -- when you make a function call, you often enter magic land. While there are no new Fortran files (company policy), you will almost certainly have to at least look at lots of Fortran in debugging it, and you may need to make code additions for bug fixes. Bloomberg is a fast paced company (due to it being driven by Finance), but this means that you focus on getting the final product done ASAP -- which can be exciting -- but rarely will you get the chance to take time and properly design and develop a product, or refactor old code that, while not technically broken, is inefficient.