Hi, I've been using KDE/X from testing for a few years, which implies I'm generally pleased enough. As others pointed out, there are problems; the one I've experienced most, in the last few weeks, is kwin crashing (it restarts automatically and windows seem to all stay where they were, so it's not terrible). I should point out that I'm not a very sophisticated user, my common mode of operation is one window maximized on one screen, while ignoring the other screen; but I do mix windows from different users in one X-session. In fact, the thing that's been broken for me so long that I've learned how to deal with it, is sound only being available to one user at a time.
Your mileage may vary, Shai.