I read it, or at least tried reading it. It's incredibly detailed, so much so that I barely got 1/3 of the way through it before just admitting that I wasn't retaining all of the nitty-gritty it had. If you have a photographic memory, it's great. I think it's best point is studying the immune deficiency diseases because those come up a lot on Step I, so you can't get enough of DeGeorge's syndrome or Burton's agammaglobinemia and whatnot, but I don't think it's worth going too hardcore on things like immunoglobin recombination, which is what HY Immuno does.
I used it along w/ my immuno course during MSI. I thought it was a pretty good supplement. I don't know how much detail you are looking to learn it in for the boards but I thought it was pretty easy to read through compared to my schools text (the Abbas/Lichtman book... the blue one...its a very good book but probably too detailed for board studying).