I've been avoiding KDE4 for a while. But yesterday I threw KDE4 onto my laptop, because it runs Kubuntu and I don't really care if my laptop breaks.
When I first tried installing it, I accidentally got KDE 4.0 somehow. That version was massively incomplete. After I upgraded to 4.1 it was very apparent how much more work went into KDE between the 4.0 and 4.1 release.
Regardless, looking at 4.1, I know I'm echoing what a lot of others have already said, but I agree: KDE4 is nice, but it's nowhere near KDE3 quality yet, and nowhere near complete. Some issues:
- The default theme is usable but bland. It's far better than some of the garish, loud, cartoony default themes KDE has had in past versions, at least. But it doesn't have a lot of life to it.
- Most icons are missing or broken (showing up as "?"). This is probably due to KDE being packaged improperly for Kubuntu, or something choking during my install, but regardless, it's annoying. Given the behemoth size of KDE, I expect there will be a lot of this kind of problem for a long time. Who knows when KDE4 will be stable in Gentoo, for example. One good thing about KDE3 is that a lot of these little bugs have been worked out already.
- Kickoff, the KDE launcher menu replacement, is no fun at all. There's way too much wasted space and you have to mouseover and click a million times to open anything. Same with the new-style control panel. You have the option of switching back to the old-style menu, which is a good thing.
- There are a lot of funky compiz-like animations that you can enable. Transparency, desaturating or tinting or dimming windows, and fading / exploding / zooming window animations etc. At this point, given Compiz and OS X and Vista which have mostly the same things and have been out for a while, these features are nothing new or mind-blowing, but it's nice to have them built into KDE if you like that kind of thing.
- Dolphin in KDE4 has some nice new features that my KDE3 version is missing, like a split-column view, and the ability to show thumbnail previews in more situations.
- The panel configuration window is gone, replaced with a neat graphical WYSIWYG kind of thing. You drag the panel edges to resize it, and click some buttons to alter the justification (right, center, left). It's nice, but after playing around a bit, when I tried to resize my panel to be 100% of the width of the screen, I had a lot of trouble; I had a pixel or two between the panel and the edge of the window. That's the problem with WYSIWYG: it's not as precise or fast as just typing "100%" into a text field.
Many KDE apps aren't ported to QT4 yet, apparently. This introduces YET ANOTHER look-and-feel for Linux. Now I have to find a GTK2 theme, a QT3 theme and a QT4 theme. There are a few unified themes (qtcurve for example) that work for all three and look essentially the same, but there are only a few and the choices are limited.
With animations disabled, when I open a menu I get a second of garbage as my video card freaks out trying to render it, before the menu appears. I can't imagine what the problem is there. KDE3 never does that on the same computer.
- Confusingly, deleting an icon from the desktop doesn't delete the file, it just removes the icon. Then I made a "folder view", which is a desktop applet that displays icons for an arbitrary folder, optionally filtering the folder by name. It's a neat idea. I'm unsure it's neat enough to replace the whole concept of a desktop full of files. Regardless, when I made one, it overlapped all the icons on my desktop to the point where I couldn't get it sorted out and had to remove it. What's the difference between icons directly on my desktop, and icons in a Folder View? It's very confusing to me, a long-time KDE user, so it's probably confusing to mostly everyone.
- The version of KDM that comes with KDE4 looks nice, but also crashes randomly.
However, my biggest problem with KDE4 is how sluggish it feels on my laptop. KDE3 runs very smoothly, but KDE4 lags left and right, when resizing windows, opening popup menus, clicking icons to open applications, or doing much of anything. It may be I need to upgrade my video card driver (I'm not sure if mine is properly hardware-accelerated), but should I really NEED to, just to run a window manager? KDE was never what you might call "lightweight" but it seems to be even further from that ideal now.
It'll be a while before I switch from KDE3 to KDE4 on my main workstation. It looks promising and there's clearly tons of potential. But it wouldn't be my first choice of environment for getting work done at the moment.