I had a window and door business for a few years. Also did a few kitchens on the side so worked on a fair number of houses and with a number of contractors. I would say a PM is analagous to a head
contractor. The ones who seemed the best to me were ones that understood what the owner had in mind, didn't have to much going on to attend to the job, had good relationships and was able to offer lots of work to the subs and new how to press them to get the work done well and on schedule which is not an easy task. The best also had guidelines. Limiting owners access to off hours was one that always helped. Change management policies that were firm and clear helped. When delivering an order of windows I would hear often from owners who weren't happy with their contractor (more often then not) and usually it was because they were over extended and deadlines were slipping. It's not an easy job. I agree with Scott that having one who can push the subs is important but it also has to be the sort of person that keeps the subs happy.