milk may be good for a kitten but it is actually not all that good for an adult cat. They are subjected to diarrhea and it can cause stomach problems such as cramps and even vomitting. Cats are lactose intolerant and should actually be given water.
However; there is a milk product called Cats Milk that is specially treated for cats so they don't get sick and all pukey.
In large quantities it is dangerous for them. To a kitten that is under the age of 8 weeks, milk is a much needed food, but, if they cannot get it from their mother then you need to go to your pet store and purchase the kitten milk formula.