I'm a current IRTA at the NIH, it definitely is a good experience to have. Lots of opportunities to network and gain lots of experience in the medical field. But I did find it hard to study and work at the same time. I had to be REALLY disciplined, as in wake up around 6 am everyday get ready and study from 7 to 9:15 am. Then go to work, come home, maybe workout some, make dinner, and study a few hours in the evening. Just be sure you can stick to this. As areserine said above me most P.Is are flexible, and just communicate with them that you are studying for your MCAT I'm sure most of the time they will be accommodating. Feel free to send me a PM if you have any other questions.