What kind of monitor and video card do you have? (My various computers are pretty plain vanilla; I've only run into one head-scratcher of a problem where X windows would hang when quitting a KDE session and going back to kdm; or switching to a console, on a box that has built-in ATI Rage 128 video. The solution there was to not load the GLX extension.)

Yes, XFree86 works fine, and it's probably more stable than X.org in Debian at this stage. (Not that X.org isn't stable, it's just that it's new to Debian.)

Some commenters I've read on the Kanotix board say that X.org (which is now in Debian Sid) is faster than XFree, and that it supports more graphics cards (and, of course, you can get nifty translucency effects). Other commenters say that some of their games won't run after they install X.org. YMMV.