Pros
I think you'd be hard-pressed to find a more dedicated group of individuals working together to further a single cause. The employees are what make the USS Constitution Museum possible - they put in an extraordinary amount of time and effort into their work with little opportunity for personal or professional advancement.
Cons
Compensation is low, even for a nonprofit. To their credit, the Museum does try to compensate with a decent benefits package, but it doesn't make up for substandard salaries in a city with a high cost of living.
Unfortunately, there's little administrative support for employee growth - employees fund professional development courses, workshops, and conferences largely out of pocket, and the organization is structured such that vertical movement is impossible.
The truly troubling downside, however, is the senior management. Nearly every person in administration has been with the Museum in excess of 15 years, and while that can be a benefit in some organizations, it's created a toxic administrative culture at the Museum. Senior management is disorganized, cliquish, and overwhelmingly non-receptive to outside input or innovation, leading to high rates of employee dissatisfaction.