Pros
The company is filled with intelligent people who work on challenging projects. The lab requires a broad range of skillsets and will give you exposure to many different areas. The biggest pro that I have seen in recent years (given the economy) is that the lab offers a high level of job security. Getting a security clearance is an expensive process and they are unlikely to get rid of you after absorbing that expense.
Cons
Although the lab is billed as a meritocracy, it is really a bureaucracy through and through. The number of employees at the associate and technical staff levels are capped so advancing to a different level is difficult. I have seen people climb multiple levels within the lab, but that is the exception. Mostly I have seen people climb one level and then stop. Promotions are difficult to come by as a result. The work can become monotonous and if you are good at what you do it may be difficult to branch out and do something else. Pay is lower than industry.