Pros
Great teams of consultants to work with and learn from. Everyone works hard to serve clients and deliver great services. Makes a great place for new consultants to get their feet wet and get involved in visible client facing projects very early (while those clients are still around), just don't stay too long. Vast data sets provide opportunity for innovative research. Looks good on your resume for now, and there's a vast network of Gallup Alumni who have already escaped and often work together at a later date.
Cons
Leadership is rife with shameless nepotism and "yes men". Employees are not rewarded for a job well-done. Most likely a job well-done will instead result in a restructured plan to pay you less and bolster profits. Science is dated and hasn't been advanced in decades. Technology is just as dated and leads to long lead times and embarrassing offerings to clients.