Pros
Compared to extramural research, there are significant benefits to working intramurally: No teaching, no grant writing and yet plentiful resources. Most labs are in the top 10% of their field. There are many institutionalized support structures for trainees, especially for those from foreign countries. The generally spirit is to get people to leave after finishing their projects quickly (2-4 years), but some people decide to and can stay longer. After 8 years it is up (meaning a cushy government FTE position as a staff scientist) or out. Foreigners are generally expected to return to their home countries, unless they can secure a faculty position elsewhere.
Cons
Compared to Ivy-League schools and other R1 institutions, you will generally sense a lack of competitiveness at NIH that is an inevitably consequence from the lack of fight over resources. Some people thus get lured into a false sense of security. Postdocs have practically no long-term perspective at NIH. It is up to the individual to not forget that they are facing stiff competition outside the sheltering walls of NIH.