Pros
MITRE offers great benefits, good salaries, fantastic knowledge sharing, flexible work schedules, and an in-house training institute (not just online self-paced training). It provides the chance to work with some really smart and dynamic people. There is flexibility to move among divisions, so if things aren't ideal where you are, you have a good chance of making it work elsewhere within the firm. The company works with leading edge technologies, so employees are provided with high-end equipment and resources. The knowledge worker is the firm's primary resource, and employees are therefore given fabulous opportunities to stay current and share information with one another. In many ways, the culture at MITRE is like the ideal of an academic research institute - people have the freedom to pursue fascinating lines of inquiry, are paid very well to do it, and get to work in a beautiful campus environment.
Cons
There are significant differences among divisions - some are more bureaucratic, some are more insular, etc, and it might be hard to know whether you'd be comfortable in a given division in advance of starting. The client-facing centers are more cost-conscious, which makes for a somewhat faster pace of work. The corporate side does not do cost recovery, which can make for a somewhat more laid back work environment, but also means your projects may take years before they come to fruition. The principle focus of the firm is on matters that relate to defense and national security, which will be an upside or downside, depending on one's perspective. The typical MITRE employee has been working at the firm for 10 years or more, so newcomers might (or might not) have a hard time fitting in.