Pros
I loved the people I worked with at Fidelity - smart, creative, hardworking, fun. Most people care about the customer and work hard to provide great service. Operations and Technology groups had many talented individuals and middle managers, but lacked a cohesive strategy and strong SVP management. I had excellent training opportunities and career progression. Fidelity was a great company for the 15 years I worked there.
Cons
With the overhaul at the executive levels from 2007 onwards, and the revolving door of external hires at the highest levels, the frequency of organizational and strategic changes became intense. Major layoffs took place in 2008 and early 2009, due in part (but not entirely) to the market turmoil. The whole financial services field has been downsizing, so think carefully before you apply for a job at Fidelity if the stock market is still very rocky. Be prepared to manage your career and build a good network. Have a "Plan B" ready in case you get laid off.