I've been using the book for the last few months just to have some variety in materials to study. It's nice to test your knowledge from the vignette point of view. It helps to compare & contrast diseases and how you might differentiate them.
I'm not really planning on using it for my step 1 study. I think I'll have more than enough with FA, Goljan, and UW.
Take a look at a copy, read through a few cases, and see if you like it. I'd say it's definitely supplementary material, though it seems to be high-yield as it is based on FA.
