Lactose free milk is in fact milk with added with lactase(an enzyme) to convert the lactose in milk into glucose, thus making it easier for those lactose intolerant people to digest the sugars easier.
From the posts above most say it would help in combating bloating and flatulence. Weird as for me, i did try lactose-free milk some time back and still experienced those symptoms. Don't know what kinda stomach i have sometimes..