I wanted to install WindowMaker because I needed a windowmanager that runs on low resources.
Also it is very nice to customize and installation is a breeze. To configure what user runs what desktop, you need to make a .xsession file in the home directory of that user. Mind the dot (hidden file) and make sure it's executable (chmod). Edit the file and put this line in it: exec wmaker. Here you can also specify some other programs to autostart or add arguments to the windowmanager. I've used the --no-dock and the --no-icons arguments to make WindowMaker start with an empty desktop. This page also has some nice WindowMaker tricks: www.gbar.dtu.dk/index.php/WindowMaker