Pros
Has internal "Bloomberg University" which is especially good for R&D folk. Many well respected technical authors come and speak from outside. Staff & colleagues are generally very smart Opportunities to learn about many areas of finance as well as technical subjects. Has a big snack kitchen and excellent fish tanks. The London office is generally very nice and gives you a sense of pride. There's not any necessity to move into managerial positions if you are strong technically and want to stay doing hands on programming. Bloomberg is very much involved in charities and encourages the involvement of it's employees in various volunteering activities. These are a great experience. The summer parties are excellent.
Cons
Hours are long and workload is large. It's not a good work/life balance. People sometimes get shuffled round against their will into different teams. Your experience in a team can depend a lot on the luck of who your manager is. Some managers are great, others get put into managerial positions because they were good programmers but don't necessarily have any people skills. Review process is somewhat opaque. Atmosphere on development floor is very quiet - you may find this a plus, I found that it doesn't give a strong sense of belonging to a team as most people are working independently. This can also lead to disjointed projects. Most programming teams have their own business department, so if you like interacting with clients on the creative side of coming up with a new product then you might find your opportunity to be involved in this a bit limited.