Some people just can't drink milk. Most often it's due to lactose intolerance, but some other people are allergic to proteins found in milk.
I've also known a few people who seem to have trouble consuming large amounts of milk fat. The easy answer to that, of course, is non-fat milk.
But for those who are able to digest lactose and don't have any allergies or other problems, milk is very nutritious.
I drink lots of cow milk. I have no trouble with it at all. But I also enjoy almond milk, which is a pretty good substitute. (I find it much better than soy milk or rice milk.) But it varies a lot between brands. Some companies put in way too much sugar for me. Some have odd flavors or textures. Trader Joe's has one which tastes like cooked almonds instead of the more subtle raw almond flavor found in most. (I like it, but it's not the same as normal almond milk.) I tend to use the unsweetened variety, but I think most people prefer it to be sweetened or flavored.