So I bit the bullet and installed KDE 4.0 on Gentoo. Version 4.0.4 recently hit the tree, and with some minor hackary to package.unmask and package.keywords I have a nice spartan KDE 4.0.4 desktop that I am typing this in.
Like Gentoo's KDE3 packaging, you are given KDE4 pretty much as it is packaged upstream. It certainly lacks the polish that Fedora 9 and OpenSUSE 11 have been able to apply.
The desktop is somewhat of an afterthought at this point. Basic icon grid alignment and drag selection don't work. Icons on the desktop are treated as widgets which means you can rotate them and such which may be something to build on in the future. However this area basically needs work.
Oxygen is beautiful. Seriously, everything looks stunning and it does not get in the way. It's not gaudy. Very professional. Still room for improvement though, I.E. icon set isn't complete, consistency.
kwin's built in composting is awesome. This is exactly the kind of thing I've been waiting for. It doesn't have the heavy weight and gaudy appearance of Compiz or Vista. This will be a major productivity boost. Tabbing through windows, expose-zoom overview, and a heads-up of virtual desktops are the things I will use. The light shadowing and animation are also refreshing and professional. Nothing to get in the way here, it is stable and speedy.
The application launcher is a major pain in the ass. The integrated search is handy, but it just takes too many damn clicks to find an app otherwise. This will be especially unwelcome by new users since you might not know if the app you want is in 'Internet' or 'Development' or 'Multimedia' or even 'More Applications' under any of these. Every time you click the menu it starts from the favorites pane and does not remember the category you were in. I don't think the Windows 95 style start menu is the final or best UI answer, but it beats this Vista style menu and the OS X dock abomination hands down.
Basically, plasma as a whole needs work. Widgets seem pretty glitchy. Also responsible for the above desktop nuances.
Apps are hit or miss. Gwenview is great for thumbing through pictures. okular does the same for documents. Dolphin works very well for most tasks, while Konqueror is there for you otherwise. Many of the games have been updated and are pretty good. Marble, Kalzium - just plain cool. Ark was one of my unsung heros on KDE3. It is severely lacking in KDE 4.0. I hope that it gets some attention, and shell integration back too! A lot of other utilities are either absent or need work as well.
Amarok 2 has yet to be released, but this will be a major influence when it is.
Speed! KDE4 is quick. This is a welcome relief. Once KDE4 starts to gel, I'd imagine it will run at least as well as KDE3 on old hardware. Build speed was much quicker with cmake, which is very welcome on Gentoo.
Stability is much improved in this release. It still is not business-grade. Trivial apps seem to crash on exit and such.
The Weird? The default window size for just about everything seems unnecessarily small. I find myself having to resize pretty much everything before I can use it.
In conclusion, KDE 4.0 is a giant leap forward but it is a work in progress...
If you are migrating from KDE3 on an old install, best wait as there is some adjustment. KDE 4.0 as packaged in Fedora 9 and OpenSUSE 11 seem quite usable however. I wouldn't hesitate to install them on a new PC. I will certainly keep KDE 4.0 installed and re-evaluate with 4.1, but KDE3 has a better work flow for my use at the moment.
It reminds me a lot of Apple's OS 9 to OS X transition. I've been following the development for some time. From the alphas, betas, and release in January, I'm quite surprised by where we are at today. Things are much better than they were just six short months ago. A lot of work has taken place and we are really forging the foundation for a first rate competitor in the PC desktop arena for the next 10 years. KDE 4.1 should be be released near the end of the summer. I believe it will be enough to convert many KDE3 users over. Things should really calm down and align by 4.2 and it will no doubt in my mind be the best desktop environment whatsoever at this point.