The build service is just an easy way to bundle software for SUSE, instead of having to add a repo and finding the right things to install, you can use a 1 click install and that will install everything you need. It works fairly well, and you can even package software for other distro’s. As for the KDE thing, if your running 10.3 and want KDE 4.0.4 (soon to be 5) you can easily install that with 1 click, You can then choose to add those repos to your list permanently, so you’ll always get the most updated packages of KDE 4.0.x (or KDE4 stable) They also have a KDE4 unstable, which is KDE 4.1 (4.0.80 because it’s not released or finished yet, it’s still beta)
The build service is great. Here’s a caution though: I would enable it as a repo in an experimental or cutting edge installation of openSUSE. But not for my working, everyday installation of openSUSE. If you update off of the Build Service repos you get packages that are unproven in the 10.3 environment. So yes they’re great but be aware of the old saying “risk is proportional to reward”.