3 weeks ago Kde-4 did not run using kdbus. I did not look up why. That same kernel for my Gentoo kde-4 works without issues since then. Although this kernel is not modular I could run it with Debian by adding kdbus=0 to the grub kernel command line.
Using kdbus with the Gentoo partition is a no brainer since systemD recognizes this facility in the kernel and a handful of dbus1 proxy processes are started, which are waiting for traditional dbus1 calls then. Look at a few:
8976 systemd-b 1 0 1469M 3152 2368 S 0.0 0.1 0:00.03 /usr/lib/systemd/systemd-bus-proxyd --address=kernel:path=/sys/fs/kdbus/0-system/bus
9009 systemd-b 1 0 1469M 3152 2368 S 0.0 0.1 0:00.13 /usr/lib/systemd/systemd-bus-proxyd --address=kernel:path=/sys/fs/kdbus/0-system/bus
255 systemd-b 1 0 1469M 3152 2368 S 0.0 0.1 0:00.94 /usr/lib/systemd/systemd-bus-proxyd --address=kernel:path=/sys/fs/kdbus/0-system/bus
936 ral 1 0 1749M 3456 2248 S 0.0 0.1 0:00.21 /usr/lib/systemd/systemd-bus-proxyd --address=kernel:path=/sys/fs/kdbus/1000-user/bus
939 ral 1 0 1749M 3456 2248 S 0.0 0.1 0:00.35 /usr/lib/systemd/systemd-bus-proxyd --address=kernel:path=/sys/fs/kdbus/1000-user/bus
988 ral 1 0 1749M 3456 2248 S 0.0 0.1 0:00.00 /usr/lib/systemd/systemd-bus-proxyd --address=kernel:path=/sys/fs/kdbus/1000-user/bus
You will not feel any difference. Which does not wonder since dbus1 - as a slow compangion - is used mostly to communicate short messages. If you look at Gnome-3, there probably is more to expect, because it is far more cut into pieces than kde-4.
The problem the kernel guys discuss most these days is about what to expect from kernel dbus code if there is a user space program with ugly intentions. I don't think traditional dbus1 code ever was secure in this case, but never was kernel code running at pid1!
For me the interesting about kdbus is: systemD could get rid of all the hidden special services which are run automatically at the beginning of the boot process. That way I could experiment with the very first boot sequences ...