Pros
Well respected name, one historically noted for rigor and objectivity in its science. Very nice offices, with pockets of smart and dedicated people in a variety of locations.
Cons
Strong belief that only "talent" matters; everything else can be readily taught in a short session or two. Generally have little value for experience-based perspective, even though that's a key ingredient in what a consultant should provide, and what a client should expect. Relentless focus on client profitability means that clients are increasingly served by the newest and cheapest resources. Gallup endeavors to be seen as Bain or McKinsey, but it doesn't recruit, staff, structure or reward as they do. No sense of corporate strategy; only a desire to somehow grow, and increase profit by cutting costs.