I was wary at first too, but then I thought about it - it's a little bit like using a serrated knife. The blades may get stuck in a hair or a particularly dense clump, and the vibrating action sort of cuts through it. For average shaving, like a lady's legs, it probably does nothing special. But on my face, after two or three days' growth, it's a serious help - I tested the regular Mach 3 side-by-side against its non-vibrating cousin, and it seriously works.
Initially, I was very concerned about it vibrating straight through my jugular, but I've discovered that's just another unfounded fear. Come to think of it, that's another question - how are you people cutting yourselves? I could see if you're using a straight razor, but with safety razors, unless you're dragging it sideways, there's basically no way to do it. I could see knees, maybe, they're kinda knobbly - but I've got the area around my nose, and the chin, and I've never cut myself. Maybe I'm just particularly adept.