I'm currently a post-bac fellow at the NIMH (mental health). I work in a brain imaging lab under a really great staff scientist. There are other research fellows like myself in my lab, and there are tons of us on campus. I've had a good experience with work and getting along with the other IRTAs (that's what we're called - Intramural Research Training Award fellows) through different activities (even the social club is called Club PCR, so you gotta know it's off the hook).
Depending on the lab you're in, and who you work directly under, can make or break your experience. The purpose of the program is to give recent college grads more experience with research, working with patients, etc, and prepare us for the next step (there are workshops on the different application processes, interviews, etc). That being said, it's good to have a laid back, understanding boss. They're really great people to talk to about the application process, or things about what your lab does, or other various science interests. My boss and my section chief are sympathetic to our plight, and give us any necessary time off from work to work on apps/go to interviews/etc. Only during crunch time (abstract deadlines) do things get really stressful because we're trying to get everything done. (The one horror story I share with others is a fellow co-worker who had a very demanding post-doc as her boss, and the boss made her do b***tch work, pretty much, and actually got mad at her when she asked for some time off before the MCAT.)
As for money, you won't be making much. (If you are in an NCI (cancer institute) lab you make slightly more than everyone in the other institutes). remember this is the federal government, and if you enter this universe you will be at the bottom of the totem pole. Your lab should be able to help you with things such as getting on abstracts to present at and attend conferences, but it'd be difficult if it was an international conference. They can also help you with attending workshops and seminars related to your research (for example, many newbies in my lab attend a weeklong fMRI course in Boston at MGH. This year, I'm thinking about trying to go to something at Cold Spring Harbor this summer). Another way to supplement your income is to participate in NIH studies as a normal volunteer.
One unfortunate thing is that they can't pay for any transportation for a personal interview. Since i was in CA, i did phone interviews, but i did interview in one NIDA (drug abuse) lab up in Baltimore when i was in DC for spring break. (That interview made me decide to NOT go to Baltimore for the time being - I wanted to go to Bethesda).
Overall, I'm having a good experience, continuing to get some clinical experience (many observe rounds in their labs, we can observe ICU rounds, and shadow physicians) as well as volunteer experience (there are tons of organizations around on and off campus), and you can take graduate level classes on campus through the Foundation for Advanced Education in Science (biochem, physics, and really upper level courses such as drug discovery, health law, advanced biostats, as well as MCAT review and foreign language classes - depending on how you can justify it, you can have your lab pay for the tuition and books).
If you have any more questions, feel free to ask, there are tons of us IRTAs around here 🙂 good luck!