Pros
There are a lot of different areas of the company which allows for a lot of opportunity. You can spend many years there and actually experience a number of different careers. Unfortunately, it takes a lot of personal career management and "political" savy to get the opportunities. Fidelity is very innovative in their use of technology. They have an amazing technology research center where they are constantly are looking to improve their systems.
Cons
Very old school culture. If you haven't been with the company for at least 10 years it feels like no one listens to you. There is also a very strong aversion to change, you hear things like "this is the way Fidelity does it" and "Ned (the Chairman) likes it to be this way". Unfortunately, I think people are afraid to question the way things are done. Also, it is a large (50k employees) private company that is mostly family owned, so trying to drive change is almost impossible.