It is easy to use and looks quite good, but the functionality of the program is just not what the average linux user requires. On ubuntu for example the package "openssh-server" contains the sshd server. Searching for "ssh server" gives me zero results in KPackageKit. In GNOME's package manager synaptic I get over 100 results including openssh-server.
KPackageKit is only useful for people who know exactly the names of the packages they want to install. And these guys will install their packages with one command line before firing up a GUI for this task.
Since it is not useful for experts and not useful for the average user, it is not useful at all. It does not matter how many package-management-systems packagekit can handle when searching for packages does not work.