OK, well I fiddled with the demo briefly, and it is in fact slower than quark 4 running in classic. However it did open very fast. It managed to open a TV book that I have done in quark 4. Everything looked intact, so that's a plus... it said my document had some minor repairs, and then fixed them. Not uncommon from quark 4.
The one thing I did like that was fixed was the zoom/type/crash bug I used to experience. This is what happens to me in quark 4: I zoom in to see what exactly I am editing. When zoomed in too close, and I type a letter - crash - quark unexpectedly quits. This is no longer the case in quark 6.
There are some funky gui display problems now and then... like the clipboard for instance. If you resize the window (minimize using the corner drag) what ever it has in it's clipboard (images/text), will become broken and not as viewable as a $899 app should be.