Pros
Gartner offers the best work-to-life balance of any company I have ever worked for. If flexible schedules, generous PTO, and high-levels of personal freedom are important to you, I'd highly recommend Gartner. The ability to work from home is a HUGE plus for me and the lifestyle that I live, and Gartner is very flexible and supportive of this as a whole. The people at Gartner are also first class: very intelligent and professional, but friendly, fun, and almost like a family as well.
Cons
They seem to undercut salaries from what I've seen in comparable jobs/positions. They aren't as generous with raises and/or promotions, and internal transitions are difficult and met with little to no raise in salary. While bonuses are nice, I would rather see my performance rewarded with faster growth and greater promotion potential.