Window Maker is a window manager/desktop environment that re-creates the old NeXt-Step GUI, and hence, it's home folder, if you use it, will show up in your directory as "GNUStep". NeXt-Step veterans (all two of them?) will feel right at home; the rest of us start at square one.
You know you're using a different desktop when you start Window Maker. There's the strange cubic icons that behave funny if you drag them around. There's the root menu that appears very solid, as if molded from plastic. There's the draftsman's-workbench feel to it. When you resize a window, it draws blueprint-like arrows along the borders with pixel dimensions in case it was crucial you get the size to an exact number. Perhaps you pulled an icon off the dock into the middle of the desktop and released it in the middle of the screen, only to see it explode into confetti like a party favor. Or maybe you turned on the clip's "magnetic" feature and then watched as subsequent icons from other programs you started daisy-chained themselves to the clip.
Then you discover dockapps. Similar to the panel applets that run in other desktop environments, dockapps are square mini-programs that can be attached to the Window Maker dock. These are divided about evenly between fun and work; they range from system monitors and useful utilities to display hacks and games. In fact, dockapps just plain rock! They can also run in Blackbox's and Fluxbox's slits. Unless your install came with them (rarely) you'll have to download them. The dock can also host icons from programs, should you place them there, and then they will be accessible just like icons on a panel.
Eventually, you click on the dock square itself or pick the customization dialog off the menu, depending how your install is configured. Here is every single thing you can set in Window Maker, from the icon behavior to the menu contents, all in one handy interface! There's even a simple GUI style-designer: simply set your desktop colors and backgrounds, and when you're done, select "save theme" from your root menu and give it a name. Many choices are possible to customize the look, including using pixmaps and gradients to color any element. It falls just short of having as much eye-candy as Fluxbox, being more in FVWM's class as far as looks go.
I would rank this one as newbie-friendly, since help balloons, once turned on, explain the whole show. And the settings application is pretty verbose; once you've hit it a couple of times, you're up to speed.
Window Maker grows on you, if you give it a chance. Many of it's quirky features, such as detachable menus which you can drag to set into the bottom of the screen and then mouse over them to make them pop up, strike one, upon reflection, as a good idea.
Speed-wise, it's more in the window manager category than the desktop environment one. Whatever lag I've experienced on a Window Maker desktop has always been due to having a lot of dockapps loaded up.
If you prefer to get under the hood, the GNUStep directory contains text files just like other window managers which can be written directly or scripted. Window Maker is extremely geek-friendly, without being geek-required.
For some reason, Window Maker just doesn't seem to get the respect it deserves. It's fast, never bombs, is easy to work with and can even be styled to look cool - what is it doing wrong? Anyway, I'd recommend Window Maker for the engineer. The whole interface says, "Let's go to work!"