Pros
They have everything right here: Dentist, bank, gym, clubs, sports, doctors, gardens, free transportation, theater. They even have staff housing if you want it. People are generally friendly and relaxed. Generous time off and benefits. The location is perfect for getting to Boston. They have a habit of promoting from within the university; even across departments. Knowledge of MIT is a huge help when applying to higher up positions within the institution. So, a lot of people start pretty low on the food chain and work their way up. I really like my boss and my colleagues. They're a pleasure to be around.
Cons
It takes a while to advance, depending on the department you're in. Budget cuts effect the university just like anywhere else. If there isn't room for you to move up, you may have to try to switch departments. HR probably won't suggest you move up, so you have to be really proactive. The size of the institution can be a little daunting; there's definitely an MIT culture that you have to learn the nuances of. Some of the buildings are pretty old and have problems with heating, air conditioning, mice, etc. There are students everywhere during the semester, so be prepared for crowded hallways and the like.