Avantages
Get to work with really bright people including other full-time researchers, graduate students and professors. If your career goal is to end up in academia, working here is definitely a resume booster. For self motivated people starting and working on pet projects (on your own time which could mean weekends) is relatively easier and this is definitely not discouraged. You can tap into the vast resources of the MIT campus including a state of the art athletic facility. As an added bonus Cambridge is a great place to work that's also hosts a large number of tech companies so networking is easier.
Inconvénients
Job is extremely dependent on the projects or research group. You could be at the bleeding edge of R&D or end up doing routine work.