Pros
The access you get to COIs because you are not viewed as a vendor is brilliant and you are surrounded by experts internally, soak up the training. If you are young and want to enhance your CV then great but outisde of that they are way better opporutnites and $$$$ to be earned elsewhere.
Cons
The one true criticism of Gartner is that it only promotes from within. After a few years as a successful sales person it is easy to become a manger. However we all know, the best sales people often make the worst managers. In the real world most Gartner managers would never get a management role, therefore the levels of narcissism to protect themselves is staggering. If you are doing well as a sales person then great but the second you start to average out, or fail slightly expect knives thrown at you. Gartner is a machine run by a robot in Gene Hall, no one can change anything. They are savage is how they get rid of people.