i did an infectious diseases rotation there last year for one month... zebras galore! i was a 4th year on a team (me, a fellow, and the attending). the fellow was also rotating through there, and he was from another ID program. he told me there was a vast difference between NIH and "real world" ID.
everything is strongly research based over there. it seemed that most people were either going to or coming from johns hopkins... in fact, one attending was in her 6th year of fellowship. i think that was even unusual for NIH.
when i was eating lunch one day, i overheard the an allergy fellow arguing with one of the attendings... he was complaining about wanting to finish up sooner, to start living his life, but the research contingency is very strong at NIH, and if it's not implicity stated, it's certainly implied that you will need to hang around a while and do more fellowship years than most other programs.
we saw "rare" infectious diseases on a regular basis. you work on a close level with the NHLBI team, because of all the chemo going on, they get all sorts of fungal infections.
interesting place to say the least. and about the loan repayment program, i heard it was $15,000 (from the one who was in her 6th year of fellowship) instead of $50,000. but this was ID, not allergy/immuno. it may be different.