I would second the advice that you should be selective in who you work with. I spent the summer before my junior year in a lab at the NIH, and I worked with a doc who was extremely cold and critical, wanted to micromanage every detail in her lab, and didn't let me participate much in her experiments. I met one of her former IRTA students, and it seems she was pretty much the same way with him. Fortunately, many of the other doctors I met there were very friendly and willing to teach. From my ( limited) experience, it seemed the more laid-back docs were those who were planning to do a few years of research and return to clinical practice, while those (like the PI I worked with) who seemed to look down on clinical medicine and planned to devote their career to research tended to be much more uptight and difficult to work with. This makes sense, as the whole atmosphere reminded me of acadamia--very competitive, those who wanted to lead their own research, get grants and promotions, etc. had to be pretty devoted, willing to spend all their time in the lab and step over their colleagues when necessary. You can probably tell I'm biased--I don't like competing with others on a personal level and prefer a teamwork atmosphere--but I think this sort of PI can make your life as a student very difficult and seriously affect the enjoyment of the experience.
Despite this doctor, I had a good overall experience at the NIH and am glad I went--I got to meet some amazing and talented doctors, participate in and learn about some fascinating research, and DC is a fun city. I met several post-bac IRTA's who were applying to med school, they seemed to feel that the experience was helping them in that process.