The problem is with the microblogging plasmoid that kubuntu ships as default. It has a little "configure" nag button which breaks with any non oxygen style. In fact any plasmoid that uses a QT widget, like OpenSocial, will have the same result. If you remove those plasmoids you can switch to other styles without a problem.
Repost from duplicate bug #433028
The problem is with the microblogging plasmoid that kubuntu ships as default. It has a little "configure" nag button which breaks with any non oxygen style. In fact any plasmoid that uses a QT widget, like OpenSocial, will have the same result. If you remove those plasmoids you can switch to other styles without a problem.