If you use KDE as your primary desktop and the version 4.6.x then it is recommended to do the upgrade which should do few "patches" and few language translation fixes as well. According to the Change-log page few bug fixes of apps such as Juk (a music player application for KDE), Sweeper related bugs (a utility to clean history and other "user traces"), KDE Pacman game related bug fixes, etc are included. Nothing major it seems though.
These updates (according to KDE developers themselves) are a part of "strengthening" the KDE 4.6x series which was introduced around January (2011) that brought significant changes to the desktop and its applications.
|The Plasma Netbook edition...|
Anyhow, if you use Kubuntu 11.04 Natty Narwhal then you'll be glad to know that the Kubuntu team has also released pre-build packages simultaneously with this KDE release. If you haven't already added the PPA, then you can install or upgrade to KDE 4.6.5 version in Kubuntu 11.04 by using the below commands in your Terminal window.
sudo apt-add-repository ppa:kubuntu-ppa
sudo apt-get update
sudo apt-get upgrade