SOME cats are lactose intolerant, just as SOME people are lactose intolerant. Few human children are lactose intolerant. As people age, more and more of them become lactose intolerant (more so with some ethnic groupls than others), and the same thing happens with cats, only apparently more so. But also, just like people, they may be able to tolerate small amounts of milk, even if they can't handle a large amount. (I know that I can handle about two tablespoons of milk without taking a Lact-Aid, but if I tried half a cup of it, I'd be in misery.) This probably explains why you get such varying opinions from people. If cats are lactose intolerant, it will show up as diarrhea.
My Purdy is lactose intolerant, but Red Cat is not. At least he didn't used to be, so I used to give him milk as a treat occasionally. However, he has developed so many allergies, that in the past couple of years, I've avoided giving him most treats. Occasionally I do give him a bit of unflavored yogurt, which is often better handled.