Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
Thanks to Tejas Guruswamy for a very good write up on build service repositories for KDE.
The following is from the kde mailing list:
If all you want is a workable version of KDE, stick with what came with the distro when you installed it. Everything else can break your system at any time with no warning because it *will* always be changing.
If you want the latest released version of just the applications (e.g. amarok, digikam) use just KDE:Backports. If you want the latest released version (including betas and RCs) of the KDE Desktop _and_ applications use KDE:KDE4:Factory:Desktop.
If KDE makes a release when KDE:KDE4:Factory:Desktop is busy, the KDE team may choose to create an additional repository (e.g. KDE:42, KDE:43) in order to package that latest release.
Once KDE:KDE4:Factory:Desktop is free again, these additional temporary repositories *will disappear*.
If you decide you absolutely must have the latest version and switch to the version specific repos, be aware that they will disappear again in a few months.