Hi, I'm not a JHU postbac-- in fact, I'm finishing up at Bryn Mawr right now-- but just wanted to drop a line since there's little presence of JHU postbacs on this thread.
I didn't apply to Hopkins because I thought it was either a 2-year program or a SMP. Or maybe they required calculus, I can't remember. In any case, JHU and BM have a few linkage schools in common, notably Penn, Brown, and George Washington. Those schools just lump them in with BM people since JHU is so small, so we randomly saw 3-4 JHUers at those schools (among like 30 of us). So I chatted with a few of them, and they seemed really nice, interesting, accomplished-- pretty much the same type of people as BM postbacs.
Unfortunately, I think it's true that the classes are cutthroat. JHUers weren't particularly eager to talk about their classes. During the George Washington visit, one JHU alum even said she was pleasantly surprised by the collaborative atmosphere at GW because that's not what she was not used to... and she added that JHU postbacs probably know what she was talking about.
I'm guessing Liza is the blonde lady I saw at GW-- she didn't come to the other schools. Jodi was present at virtually every school though. She definitely has tight connections with all the schools and is clutch when it comes to linking (you have to do your part, of course). Admittedly I was pretty jeaous when I found JHU has linkage to Cornell, but it was added to BM's list this year. I was also somewhat jealous of the cool research opportunities at Hopkins, though how the postbacs find the time to get involved, I do not know.
As someone said earlier, it probably looks good that the Hopkins postbacs put themselves through the pain of studying among Hopkins undergrads. I think adcoms are impressed by the fact we pulled off this sheer madness (on top of our unique background). The cutthroat undergrad classes bit is pretty minor compared to that, and I'm personally glad to have avoided it. But I can definitely see how Hopkins would be attractive to people who want to be in a smaller program within a cutting-edge environment.
Oof, didn't mean to ramble so much, but hope that helps!