I'm using KDE4 on Debian Stable and while it's otherwise OK, the K-button menu is usability-wise just awful.
I boot & shutdown the computer daily. Shutting down the machine from the KDE4 menu works like this:
1. click K-menu button
2. move mouse a large distance to right + a bit up
3. move mouse up to select Shutdown
3b) If mouse went a bit left, over Recents icon, restart from 2)
3c) if mouse went a bit right, out of menu, restart from 1)
5. Move mouse to the center of screen and click Shutdown
(or wait the countdown)
6. Cancel redundant KWallet popup that never came on front
Bug 6) is hopefully fixed in newer KDE4 releases. The cases 3b) and 3c) are really annoying. If I click to get K-menu open, why the heck it closes without a click?
Secondly, I have *a lot* of applications installed that I use rarely. So I don't remember what are their names (so that I could just type their name) or in which submenu they are.
Instead of just moving mouse over the submenus like I could do in KDE3, now I need to click open each of the submenus and their submenus, and then click back buttons to get back. This is really tedious. Several dozens of clicks to launch some rare app instead of just moving mouse a bit and doing a single click. A real design win.
While "lots of apps" is probably a corner case, for many people shutting down the computer isn't.
Btw. Note that from keyboard the shutdown works OKish, one can use Ctrl-Alt-Del and arrow keys. The dialog coming up with that just looks crap, icons are half outside the buttons and clipped by dialog borders (this may be affected by the dialog layout being buggy in regards to sizing caused by localization).
If many of the KDE apps weren't so nice, I would use XFCE, maybe even without compositing to get a bit more speed. :-)