Make sure it's really what you want before going into the organization
Pros
The pros to working at Gartner would be the general knowledge you gather from the day to day. IT Research and advisory is a good place to get a basic understanding of a lot of areas of technology, as where as where the market's headed. So, it can be beneficial. If you are an experienced professional though, it's not a place for you, unless you really get what you request from an income perspective. That, comparatively to other "technology" companies, is not competitive. It is a place to go work if you're just getting your career started...
Cons
It all starts from the top. A CIO turned CEO, who yes, has really helped stock prices, and was a great find for the company…9 years ago. What he did though, was create an environment (through his direct reports), that is in no way, shape, or form driven by the ideas, or feedback from the hundreds of salesmen and women who actually help that company make its money. If you want to see a company that is struggling across channels when interviewing, ask for the attrition rate… Innovation is at an all time low, and so is employee morale. Not to say that every senior exec is terrible, that is absolutely not true, but if you are the type of person who likes to be around fresh ideas,..then you need to seriously consider if this is the right opportunity for you. It's an old cliche that bad managers in turn hire bad managers. Gartner is racked full of very below average leadership. Who, in turn, hire below average employees. Which leads to a below average organization over time, business unit, by business unit. Before you take the job ask them - what are your metrics for success, and how many of them are there? ask them what the attrition rate over the last 4 quarters have been ask to speak with members across the floor about morale, and management ask them the average age of the group you're entering ask them bout the comp plan, and the detailed specifics of how you get paid (the more complicated, the worse off you are) That should be enough for you to make a good decision. I do think though, that the golden days of Gartner's reign are coming to a close...