Pros
You work with talented and smart people. You would be at the forefront of technology and will know about what vendors and clients are doing. You will be in a position to impact the market; recommend strategies and talk and deal with decision makers in the industry. You will be in a position where you learn and set the trends and drivers of the market.
Cons
Managers are really analysts promoted to management positions with no managerial skills. The measures of success are pretty vague. Often you are left to yourself to determine what areas to cover without much help or direction from your manager. You work alone most of the time. As long as you deliver your reports on time; no one really takes the interest in your career development. Promotions and recognitions don't happen as one deserves. You are put in a pool with other people and it is up to your manager to fight for your rights in management meetings. I would say Gartner's promotion and reward system is their down side. Pay is also on the low end of what industry offers.