Pros
Opportunity to work with a lot of knowledgeable people; Chance to help keep the US and allies safe from adversaries.
Cons
Very challenging finding work; limited opportunities to obtain necessary security clearances to get work. The reason MITRE is a trap is simple: finding work. Even if you have long-term work established when you join, there's no guarantee that you'll be able to keep it. Once off a project, it's up to the employee to "use their network" to find additional work. This may be easier when you are junior (and less expensive) but becomes progressively harder as you advance. Thus the trap: to stay gainfully employed you need to keep your level (and hourly rate) low. That means employees have to decide between career advancement or having steady work. In short, if you stay more than 36 months, you're most likely trapped at that point.