We went the week before Christmas this past year. We were able to walk onto most of the rides up until about the Thursday afternoon. MGM did tend to get crowded in the late afternoon/evening as people came in for the Osborne light display, but other than that the attendance seemed very light.
Please note than it can be cold that time of year. We wore shorts on day one, pants and a light jacket on day two. By the third day a cold snap was in full force. We wore our heavy winter coats (we're form Michigan) with the hoods up and the kids wore two sets of pants to keep their legs warm. It was breezy, and the lines are generally shaded so it felt pretty chilly. We saw tons of people who had only packed shorts and T shirts. Some bit the bullet and spent a fortune on warm clothes at the park. Others ruined their vacations by staying miserably cold.
Have a great trip.