A study on ease-of-use between Windows XP and Linux (with KDE 3.1.2) returned some interesting results:
The study findings suggest that it's almost as easy to perform most major office tasks using Linux as it is using Windows, which has a long history in the consumer market.
Linux users, for example, needed 44.5 minutes to perform a set of tasks, compared with 41.2 minutes required by the XP users. Furthermore, 80% of the Linux users believed that they needed only one week to become as competent with the new system as with their existing one, compared with 85% of the XP users.
This sounds about right to me. KDE has come a long way, but it's still easier to to do some stuff in Windows (GUI file management, desktop settings, etc.) than in Linux.