I have just tried the version of KDE4 that can be installed through Kubuntu 8.04 (It says it's 3.3, but that seems a bit odd). Anyway, I have the following to say about it, for anyone who is interested:
I like the new 'Plasma' applets, especially the way that you can put them on the desktop - I have a nice big clock in the middle of my screen, a bit like (but much better than
) Windows Vista's little clock thing. I think this sort of thing is a necessary step to keep Windows converts comfortable with the GUI while they ease themselves into Linux.
What I don't like is the way that the desktop is now no longer like a regular folder. This is likely to be a far bigger stumbling block for 'newbies' than the lack of the nice Vista clock. I like (sometimes) to save stuff to my desktop and work with it there, or to copy and paste things, and I'm pretty sure that I'm not alone in this - many browsers default to save to the Desktop.
I like the thought that has gone into Dolphin, in producing a dedicated file manager, but as far as I can see, it isn't much different from Konqueror, but is lacking some of the useful features that Konqueror had. I would prefer to use Konqueror to manage my files, is there an option somewhere in KDE4 to do this? (I know there was in KDE3 when Dolphin came out, but I haven't quite found my way around KDE4 yet enough to do this.)
I had a lot of 'Input Actions' programmed in my KDE3 profile, but these don't seem to have transferred across, and neither can I find the KDE4 control panel.
In conclusion - this is a very nice, slick piece of work, but it has a long way to go before I will consider it a serious replacement for KDE3. This is not, however, a bad reflection on the developers - it certainly has potential - KDE3 is probably the best desktop environment I have ever worked with, and I've worked with quite a few, keep up the good work.
The wisest thing that Microsoft did when releasing their fancy new look interface for XP was to have an easily accessible option that made everything look like 'Classic' windows. With these thoughts considered, KDE4 will be the undisputed champion of the Linux desktop, and I look forward to using it in the future, but for now, I will continue to use KDE3.