Pros
There are lots of opportunities for someone just starting their career in science/research to network with some of the top professionals in their field (NIH is so big, there is definitely someone here who is doing what you are interested in), expand your interests (lots of seminars on all types of things), and expand your skills (lots of workshops on everything from new microbiology techniques to how to interview for dental school, discounted classes taught by postdoctoral fellows here).
Cons
However, it is sort of a crap shoot with your research experience though. It is hard in the application process to figure out what the lab culture is before you start because most of it is online. In general, I don't think the application process lines you up for a great experience. Online, you have to post your resume, research experience, classes and grades from school, a cover letter, and three letters of recommendation. After you have all of this online, you can begin reaching out to PIs at NIH. However, the vast majority of PIs are not hiring post-baccs currently or ever. There is no list of which labs or institutes have openings for post-baccs. So most of this process is a lot of rejection, or complete radio silence from everyone you've reached out to. It leaves you feeling pretty desperate (and chances are the date of college graduation is looming nearer and nearer everyday and all you want is a job before that day). At least in my experience, I was having so much anxiety I took the first job I was offered, without many questions about the lab, structure, culture, and it hasn't left me very happy. I would recommend to anyone applying to this program to be sure that the lab you accept is a lab you can see yourself happy doing full time research for a year or two years. This is probably common sense to others, but it didn't really hit me until I got here.