Pros
NIH is a stimulating and purpose-driven place to work. The vast majority of staff are highly committed to the mission of "turning discovery into health." There are so many interesting educational opportunities -- every day there are lectures and events. There are good employee resources as well. We utilize NIH daycare (POPI, which is *excellent*) and when my son was tiny I was able to receive lactation support in my building. There is a gym in my building that offers fitness classes, and every year they give flu shots free of charge to employees. The work itself varies a lot; for me this is a positive thing. I get to read about so many interesting scientific projects and interact with researchers around the country who are just as committed to the mission of doing health research as people on campus. It's truly a great place to work. In addition, NIH takes diversity/equity/inclusion and harassment seriously, and they have a fair system in place for giving out raises/bonuses/promotions.
Cons
NIH is a part of the Federal government, and this means Feds and contractors are subject to shutdowns, pay freezes, etc. In addition, issues with budget can make it hard to effectively move the mission forward. Pay is fair but most higher-level employees could be making more in the private sector. Benefits are good but there are a few caveats to this. The biggest one for me is that there is ZERO parental leave. This is true of all government agencies but it was something I wasn't aware of starting out. Unpaid FMLA is OK and people can donate their vacation time to you, but overall you are pretty much on your own if you have a child. Other issues with working in government include the Hatch Act which limits the types of things Federal employees can say/do during an election.