I spent about 5 years at the NIH starting in Dec. 2001 until Jul 2006.
I started as a post-bacc IRTA for 2 years, then went to tech IRTA for 1 year, and then became a Kelly contractor for a little over a year(this was all with the same investigator).
Alas I had to leave because I think I had overstayed my welcome with the intramural research program since your time there is only supposed to be for 2 years maximum. It was all for the best anyways since it was definitely time for me to go to med skool and move on with my life, but I do miss it sometimes when I look around at the extramural world of research.
I was just running the scanner for fMRI, but you'll find that if your boss is okay with it, you can easily go out and expand your horizons by taking classes(such as premed pre-reqs that you never took) or even learning about other fields entirely like MEG, PET, or biostatistics. I got the chance to get involved in a lot of projects with a lot of collaborations which also gave me the chance to learn alot of stuff that didnt directly relate to my job. And if your boss really likes you and trusts you enough he/she will let you present a poster at a conference and let it blossom into a paper.
Basically, I think the IRTA experience is first limited by luck, then your mentor/coworkers, and then how much diligence and interest you put into it.
While the pay left something to be desired, everything else about the experience was awesome, but that was because my investigator/mentor/boss was the $hit, my coworkers were chill, and the work we were doing was really interesting. I would definitely say that I got lucky in finding a group that meshed so well with my own personality and interests. I know many others who have had more negative things to say about their NIH experience either because of boss/coworkers/pay/work.
Just remember that if you're type A, try not to get a mentor or group who is more type B. Find someone who is your type because you'll want to size them up to ensure a good fit. I know that's seemingly impossible to gauge in your 5 to 6 hours there, but if you have any powers of observation whatsoever then you should be able to get a good/bad "vibe" or feeling after meeting with everyone and asking questions like:
What are your hours, what kind of work are you currently doing, what is mentor's managing style, how would you rate your job satisfaction, do you get chances to publish, etc.
Obviously you're not going to ask those exact questions, but you should definitely not shy away from asking about things that matter to you because if you dont and you wind up in that same office that doesnt share your values, then it might be just as bad(if not worse) than not getting the position at all.
I've seen many disgruntled IRTAs burn some serious bridges on their way out and you dont want that either if you plan on advancing in a related field.