I had lesser problems recently at a Conrad and filled out a comment card with the details. When I heard nothing from the hotel, I contacted, in my case, the Diamond Desk at hilton honors, outlining the problems. Within a few days I received an email from the general manager saying he would look into the problems and offering me one night in a suite when I return.

I think the issue with the comment cards is in most cases you have to give themto the staff (usually no separate boxes) so whether or not they go anywhere depends on the staff who received it. In my case that same staff was part of the problem.