Vantaggi
Mitre is a friendly place to work and has the same pluses as many other government related jobs. There is a high level of job security, which means you have to try pretty hard to be fired. You're not expected to work any more than 40 hours a week which leaves you time for doing things outside of work. The benefits are better than average and they have a very good retirement plan. They offer around 24 paid vacation days a year for a new hire (more after 5 years of service) and they are very willing to let you take these vacation days whenever you want.
Svantaggi
There are really two major downsides to working at MITRE. First, you work for a non-profit organization that is not allowed to make or sell products. This means that you end up working on small prototyping efforts at best (dealing with government contractors at worst) that are never allowed to mature into something you can be proud of. The second major downside is the politics and bureaucratic mess that the government makes. The politics are frustrating because you find that there are many people in charge not because they're good at what they do, but because they knew the right person or worked in the right branch of the military. This leads to many frustrations if you're just trying to solve challenging technical problems. Much too frequently funding is cut or "redirected" because someone new in a higher up position simply can't understand why your work would be important or feels that you aren't branching out enough and including enough people from other departments or MITRE's off-site offices for your work to be valuable. The bureaucratic mess isn't even isolated to management or projects. If I need to order something for work as simple as a battery, mouse, etc. the expected turn around time is 4 weeks. This is not something that's really good for productivity.