Pros
The original Fiserv HQ has great facilities (gym and cafeteria) and many dedicated, cohesive, friendly people who have worked there for years. I am less familiar with First Data. In general, the people that I have dealt with were competent, productive and hardworking (but more driven, less people-focused).
Cons
The merger with First Data was a culture shock to Fiserv employees. From the first All Hands meetings, the tension between managers was visible - they had very different visions and priorities and could not agree on a unified mission. Over the next year, this disjointedness trickled down. Long term employees were told that they had to move cross-country to keep their jobs. People who had worked remotely for years were told that they must drive up to an hour each way daily. They installed employee surveillance software called Sapience that monitors each keystroke, the applications used, who a person chats with etc. Some were let go, others like myself, fled to new opportunities.