I would say that 75% of the Marriott properties I have stayed at have treated me wonderfully.
However-there are properties which seem to have real problems either finding the "right"employees or which have an in house culture that breeds hostility/contempt for the guest.
I always communicate with a follow up letter to the General Manager-and if the problem was really bad a letter to Marriott HQ as well.
I also write Marriott HQ when the service recived has been extraordinary.Having been on the other side of the situation I know how much those positive letters mean when evaluation time rolls around!