I partially agree with wizenpub, but...
this is for QuarkXpress 8.0, 7.0 did not stand a chance against InDesign CS3, and I would wait for CS4 before making comparisons, yet.
I used mostly Quark up to 6.0 and I switched to InDesign with CS2 and up to now (CS3) is the main application for DTP here. Although I use Quark (6,6.5,7,8) for 3rd party projects that come here for corrections (mostly aesthetic), I cannot say I see any reason to go back yet.
With CS4 I believe this is going to be harder.
Meanwhile I have to mention that InDesign has better support for languages up from the start, no extra localization needed.
Here in Greece QuarkXpress supported Greek correctly only for Mac OS from version 4.0 and up, whistle InDesign (from CS2) does it out of the box in both platforms. Maybe if I never switched to Windows (1996) I could have stay with Quark. But then again, I am back to the Mac 2 years now and Greek is better in InDesign there too, so....I am staying with Adobe for now!
I guess will have to wait and see.
By the way I'd like to see Quark flourish and even some of the open source apps, because having more choices it is always better for the consumer and the technology!