Well, updates in Arch are to be given special attention. While it's true breakage can happen, most of the time it requires just a little manual intervention to address the problem. (such as merging changes from .pacnew files into your configs), paying attention to Arch news about updates. I've found that when a nasty bug creeps in, usually it is fixed very quickly and comes through as an update. Regardless, in both situations, the solution is often as easy as downgrading until the next update.
Originally Posted by devius
I've only had one time where an update(s) did some harm, that i had little control over. i think it was xorg-server 1.10/11 maybe, and there were some issues with my wacom and gesture recognition software. by the second update after breakage, it was fixed. i just stuck with the last working xorg-server, until it was worked again. (that was like 4-5 days later).
Other things that i commonly have to deal with on updates (which i don't consider breakage) is having to do things like recompile vmware modules, and re-install nvidia whenever i upgrade my kernel.
As far as KDE, i don't use it. too much bloat and stuff that i don't really need, so i can't really comment, in any depth, as to how well the KDE experience is in Archlinux. I use Compiz/Gnome3 (without GDM, gnome-panel and some other gnome components) on both my Arch installs. it seems to work well, and complexity is reduced by only using / running the components i need.