The KDE project received a lot of critique concerning their decisions to do without more beta and rc version of the kde 4 system. So they will publish kde 4.0 in the current state on January the 17th.
The critics are that the new version of the desktop environment system is not fit enough for a 4.0 release. People are aware of missing features and stability in the new version.
… KDE 4.0.0 is our “will eat your children” release of KDE4, not the next release of KDE 3.5. The fact that many already use it daily for their desktop (including myself) shows that it really won’t eat your children, but it is part of that early stage in the release system of KDE4. It’s the “0.0” release. …
So if you are eager to try the new version. Do it. There are no showstoppers. But if you are using kde in a productive system and you want no experiments, stay at kde 3.5.8.
Big changes and new features will arrive in the kde 4 versions that will come. But bugfixes will also being developed for kde 3.5.8.
So as always, there is no ONE need for users. Clarify your needs and decice. I think the kde team decided the correct way.