Nowadays I'm a serious Compiz junkie, so I don't think I'll be switching back any time soon. Personally, I wouldn't want to live without maximumize or the scale window title filter. However, I can definitely see why one would want to: WindowMaker is lightning fast, as well as being very simple and streamlined. When I do pair-programming that needs tools that won't run in Screen, I spin up a WindowMaker session in a VNC server. Sharing my whole gigantic screen with all the whizzy effects is impractical over anything slower than a local 100 megabit connection.
One of the problems with switching to a different window manager for your main session these days, however, is that things unrelated to window management stop working. Your keyboard settings no longer apply, your media no longer auto-mounts, GTK ignores your theme, your media keys stop working, and your panel disappears, along ever-so-useful applets like Deskbar and the NetworkManager applet.
But, this need not be so. GNOME will happily accomodate an alternate window manager. All you need to do is make sure that WindowMaker and Nautilus don't fight over the desktop, and then tell GNOME to start WindowMaker.
Of course, your desktop won't be quite as lean as if you'd eschewed GNOME completely. It's up to you to decide whether these features are worth a few extra megabytes of RAM.
First, run gconf-editor and turn off "/apps/nautilus/preferences/show_desktop". This should make your desktop go blank.
Next, you need to go to "System → Preferences → Sessions", and hit "add" on the "Startup Programs" tab. Add an entry for WindowMaker:
Now, all you need to do is log out! You will, of course, want to tweak your panels a bit when you log back in, but that part's easy: right-click and season to taste.