I think I'll never be happy with any given desktkop.
When I used to use a desktop, I used kde, but it was way too limited for my needs. Let's better not talk about gnome or xfce then...
I always liked the kde technologies better (kio-slaves, kparts and so on), and that's why I use kde programs mostly, but under fvwm. I don't really care much about which toolkit is used to write a given program (unless it's wxwidgets, which I find a bit too much buggy). For the rest, I'd just use whatever fulfills my needs, I don't feel religious about any toolkit at all.
Kde is full of nice concepts, but it doesn't suit me as a desktop. Indeed, I don't think that the desktop environment concept as a whole fits me in any form, might it be named "kde", "gnome", "xfce" or whatever you prefer.
I usually hold all my windows maximized because I have the theory that a window that you can't see is useless. So, I usually have a window on each workspace, and I don't use taskbars. Kicker is the most useless piece of software that kde has *for me*. And I don't use desktop icons for the same reason that I don't like having windows below other windows: I can't see them, therefore, they are useless to me.
This is why, while loving most kde stuff, I really don't care much about the new kde look, with a desktop full of plasma widgets. My screen is a tool I use to work, and I can't spare 25% or its surface to display shinny things. I think it's highly inefficient.
I suppose it's just a matter of taste and habit.
I am really more worried about some other things, like the final speed and responsiveness of the applications which right now seems a bit quirky. Or the future of konqueror (khtml? webkit?). The kde4 frameworks like phonon, some new programs like okular and the time line, worries me far more than the plasma desktop will ever.
I don't see kde4 becoming really usable soon, but, again, "usability" means different things for each one :P