I have used the IV's a little bit. My opinion is they are rather easy. Good for tying some things together, bad in terms of question difficulty reflective of the real thing. I find myself not focusing or thinking as hard because the answers sorta come to you easier when its several questions on the same vignette. That also makes your scores better anyway; if you can kinda pin down the disease, and there's five questions on it, odds are you might get most of those five right.
I bought UW, and I like the difficulty of those. Pretty challenging, but doable if you think.