I've stayed at two Design properties - one in Amsterdam and one in Taipei. Both were decent stays, but nothing additional was provided beyond a slight room upgrade (better view in one, corner room in the other). No free breakfast, no late checkout, etc. (though, at one of them, I didn't ask as I had to leave in the morning anyway). I would not go out of my way to stay at a Design Hotel unless I run out of other properties to try or the property is legitimately interesting in its own right.