I am an E16 guy.
I've tried all WMs and DEs under the sun. I've been trying them all for quite a while, since GNOME 1.2 days for sure (some before that, back when the choices were more like "fvwm or afterstep?" but quite consistently since GNOME 1.2)
KDE4, in my experience, on my hardware, is crashy. I've tried running it "just to see" and found that my *entire system* locks up to the point where even a remote ssh+kill of X won't recover it. Even when it doesn't lock completely I see occasional inexplicable crashes.
KDE4 is resource-hungry. Nevermind RAM usage, which is not great, I find that it spins up my CPU and leaves it there. I routinely note high CPU usage from KDE applications and background services. Nepomuk-related things are certainly not good in this area. Often I cannot make the CPU hogging go away without logging out of KDE (and sometimes not even then!)
KDE4 isn't configurable enough. I know GNOME people are boggling at this, but I like to configure my workflow "just so," and KDE doesn't fully allow this. GNOME 2.x was worse, GNOME 3.x is far worse, and Unity... nowhere close. I say this to be fair, but if KDE4 doesn't let me make it work "my way" (and it doesn't) then it's not much good. Example: It only supports multiple desktops, no virtual desktops. This is bad, because I only like virtual desktops. I can't configure which mouse button switches desktops vs. drags windows on the pager. I can't, or can't figure out, how to reduce the panel down to just the pager. I could go on, there are a lot of little details.
KDE4 doesn't do keybindings. KDE3 had some kind of solution for this, but KDE4 is a mess. The only real option is bbkeys, which works but is a stupid kind of a solution. Technically this is a "not configurable enough" problem, but it's so huge that it deserves its own bullet point.
I want to like KDE. I promote it to less adventurous users! But, when my system doesn't work how I need it to work, doesn't have a way to control keyboard shortcuts, keeps my CPU humming at 100%, eats my RAM, grinds my disks, crashes my apps and hangs my computer... it's kind of a non-starter.
I use E16. I've been following the development of E17, which is cool-looking, but until it's "done" I don't really see the incentive to switch.
E16 is stable. In the last 10 years I have seen exactly THREE bugs in E16, one of which is arguable, one of which requires $HOME to be out of disk space, and the other of which is not a showstopper. Crashes? What's a crash? My WM *never* goes down, and I am one of those insane people who has X uptime measured in months, not days or hours. Current score: 9 months. E16 is still running, not leaking memory, not eating CPU... it Just Works. I just can't justify switching to any environment unless it can approach this level of stability.
E16 is lightweight. That's kind of ironic for a WM which was known for "pretty but resource-intensive" in its infancy, but what was resource-hungry in 1998 isn't so bad any more. By being just a WM and some applets, all of which are optional and easy to disable, the complexity is low and the footprint is tiny. Even with all of the "cool" effects enabled the CPU and RAM cost is miniscule (and I don't enable them, because I prefer functional to flashy.)
E16 is somewhat configurable. Okay, so the theme you pick matters a lot (bluesteel here, for about 10 years now) but the *behavior* of the WM is all configurable from user-discoverable GUI settings panels. It's not as crazily flexible as, say, sawfish, but it has more than enough for me. Moving windows should show outlines of the new position (with guide lines to the edge of the screen!) and exact pixel dimensions and coords in the corner. Do you care whether iconified windows are shown in the alt+tab list? I don't want them, but if you do you can have them. I like to have 4x16 virtual desktops, but the option is right there if you prefer 4x4 multiple and 4x4 virtual.
E16 has e16keyedit, a crude but effective Enlightenment keybinding management tool. between this and eesh you can control pretty much anything you want from the keyboard.
I could describe the virtues of E16 for many more paragraphs (I find, for example, that E16 is one of the few modern WMs which makes using GIMP painless), but I won't. It's simply the right thing for me and, even if it weren't, I'm used to it. Improve KDE all you like but I won't use it until it gives me something I need that E16 doesn't; chances are E17 will get there first, though, so don't expect me to be switching any time soon.