Pros
Experience varies widely based on groups. On the tech side PI is one of the most modern whereas FI is stuck in the early 1990s. My group is excellent and for Fidelity we are on the cutting edge. Work life balance is great - in general the management is very flexible. There are politics but a good manager will shield the developer from this. If you can prove your valuable by innovating Fidelity will take notice - if you do nothing new then you will stagnate. Base compensation is on the lower end of what a developer could make elsewhere but the wide array of benefits, especially the contributions to 401k (7% match and generally 10% profit sharing) are unheard of. Senior management seems to have a clear vision of what they want but the execution on it by middle management is variable.
Cons
So much of Fidelity's tech is old and maintaining, enhancing the existing systems and building new capabilities requires lots of investment which up until the year the company had been pouring money into. In my opinion some of this money was wasted on buying expensive vendor products from the likes of IBM and Oracle that could have been done without paying for a license. Another issue is Fidelity still hires too many useless contractors and unnecessary project managers.